diff --git a/src/__tests__/auth.service.test.ts b/src/__tests__/auth.service.test.ts
new file mode 100644
index 0000000..578cae4
--- /dev/null
+++ b/src/__tests__/auth.service.test.ts
@@ -0,0 +1,49 @@
+import { describe, it, expect, vi } from "vitest";
+import { verifyAccessToken, hashToken } from "../services/auth";
+import jwt from "jsonwebtoken";
+import { config } from "../config/env";
+
+describe("auth service", () => {
+  describe("verifyAccessToken", () => {
+    it("returns null and logs error for invalid JWT", async () => {
+      const consoleSpy = vi
+        .spyOn(console, "error")
+        .mockImplementation(() => {});
+      const result = await verifyAccessToken("invalid-token");
+      expect(result).toBeNull();
+      expect(consoleSpy).toHaveBeenCalled();
+      expect(consoleSpy.mock.calls[0][0]).toMatch(/JWT verification error/);
+      consoleSpy.mockRestore();
+    });
+
+    it("returns null for expired JWT", async () => {
+      const consoleSpy = vi
+        .spyOn(console, "error")
+        .mockImplementation(() => {});
+      const expiredToken = jwt.sign(
+        { sub: 1, username: "test", role: "user" },
+        config.jwt.secret,
+        { expiresIn: -1 },
+      );
+      const result = await verifyAccessToken(expiredToken);
+      expect(result).toBeNull();
+      expect(consoleSpy).toHaveBeenCalled();
+      consoleSpy.mockRestore();
+    });
+  });
+
+  describe("hashToken", () => {
+    it("produces deterministic SHA-256 hex output", () => {
+      const t1 = hashToken("hello");
+      const t2 = hashToken("hello");
+      expect(t1).toBe(t2);
+      expect(t1).toMatch(/^[a-f0-9]{64}$/);
+    });
+
+    it("produces different hashes for different inputs", () => {
+      const t1 = hashToken("a");
+      const t2 = hashToken("b");
+      expect(t1).not.toBe(t2);
+    });
+  });
+});
diff --git a/src/__tests__/customers.schema.test.ts b/src/__tests__/customers.schema.test.ts
new file mode 100644
index 0000000..3ac8261
--- /dev/null
+++ b/src/__tests__/customers.schema.test.ts
@@ -0,0 +1,19 @@
+import { describe, it, expect } from "vitest";
+import { UpdateCustomerSchema } from "../schemas/customers.schema";
+
+describe("UpdateCustomerSchema", () => {
+  it("rejects empty name", () => {
+    const result = UpdateCustomerSchema.safeParse({ name: "" });
+    expect(result.success).toBe(false);
+  });
+
+  it("accepts valid name", () => {
+    const result = UpdateCustomerSchema.safeParse({ name: "Acme Corp" });
+    expect(result.success).toBe(true);
+  });
+
+  it("accepts partial updates without name", () => {
+    const result = UpdateCustomerSchema.safeParse({ street: "Main St" });
+    expect(result.success).toBe(true);
+  });
+});
diff --git a/src/__tests__/env.test.ts b/src/__tests__/env.test.ts
new file mode 100644
index 0000000..83b4b85
--- /dev/null
+++ b/src/__tests__/env.test.ts
@@ -0,0 +1,34 @@
+import { describe, it, expect } from "vitest";
+import { config } from "../config/env";
+
+describe("env validation", () => {
+  it("has numeric port within valid range", () => {
+    expect(typeof config.port).toBe("number");
+    expect(config.port).toBeGreaterThan(0);
+    expect(config.port).toBeLessThanOrEqual(65535);
+  });
+
+  it("has JWT_SECRET defined", () => {
+    expect(config.jwt.secret).toBeTruthy();
+    expect(config.jwt.secret.length).toBeGreaterThanOrEqual(32);
+  });
+
+  it("has TOTP_ENCRYPTION_KEY defined", () => {
+    expect(config.totp.encryptionKey).toBeTruthy();
+    expect(config.totp.encryptionKey.length).toBeGreaterThanOrEqual(32);
+  });
+
+  it("has positive JWT expiry values", () => {
+    expect(config.jwt.accessTokenExpiry).toBeGreaterThan(0);
+    expect(config.jwt.refreshTokenSessionExpiry).toBeGreaterThan(0);
+    expect(config.jwt.refreshTokenRememberExpiry).toBeGreaterThan(0);
+  });
+
+  it("has positive maxUploadSize", () => {
+    expect(config.nas.maxUploadSize).toBeGreaterThan(0);
+  });
+
+  it("has DATABASE_URL defined", () => {
+    expect(config.db.url).toBeTruthy();
+  });
+});
diff --git a/src/__tests__/exchange-rates.test.ts b/src/__tests__/exchange-rates.test.ts
new file mode 100644
index 0000000..4ea603a
--- /dev/null
+++ b/src/__tests__/exchange-rates.test.ts
@@ -0,0 +1,64 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { toCzk, getRate } from "../services/exchange-rates";
+
+// Mock global fetch
+const mockFetch = vi.fn();
+global.fetch = mockFetch;
+
+describe("exchange-rates", () => {
+  beforeEach(() => {
+    mockFetch.mockReset();
+  });
+
+  describe("toCzk", () => {
+    it("returns amount unchanged for CZK", async () => {
+      const result = await toCzk(123.45, "CZK");
+      expect(result).toBe(123.45);
+    });
+
+    it("throws for unknown currency when API fails and no cache", async () => {
+      mockFetch.mockRejectedValue(new Error("Network error"));
+      await expect(toCzk(100, "XYZ")).rejects.toThrow(
+        /Nepodařilo se získat aktuální kurzy/,
+      );
+    });
+
+    it("throws for unknown currency even when API succeeds", async () => {
+      mockFetch.mockResolvedValue({
+        ok: true,
+        json: async () => ({
+          rates: [{ currencyCode: "EUR", rate: 25, amount: 1 }],
+        }),
+      });
+      await expect(toCzk(100, "XYZ")).rejects.toThrow(/Neznámá měna: XYZ/);
+    });
+
+    it("converts EUR using fetched rate", async () => {
+      mockFetch.mockResolvedValue({
+        ok: true,
+        json: async () => ({
+          rates: [{ currencyCode: "EUR", rate: 25, amount: 1 }],
+        }),
+      });
+      const result = await toCzk(100, "EUR");
+      expect(result).toBe(2500);
+    });
+  });
+
+  describe("getRate", () => {
+    it("returns 1 for CZK", async () => {
+      const result = await getRate("CZK");
+      expect(result).toBe(1);
+    });
+
+    it("throws for unknown currency", async () => {
+      mockFetch.mockResolvedValue({
+        ok: true,
+        json: async () => ({
+          rates: [{ currencyCode: "EUR", rate: 25, amount: 1 }],
+        }),
+      });
+      await expect(getRate("XYZ")).rejects.toThrow(/Neznámá měna: XYZ/);
+    });
+  });
+});
diff --git a/src/__tests__/invoices.service.test.ts b/src/__tests__/invoices.service.test.ts
new file mode 100644
index 0000000..58931d0
--- /dev/null
+++ b/src/__tests__/invoices.service.test.ts
@@ -0,0 +1,55 @@
+import { describe, it, expect } from "vitest";
+import { invoiceTotalWithVat } from "../services/invoices.service";
+
+describe("invoiceTotalWithVat", () => {
+  it("calculates subtotal without VAT when apply_vat is false", () => {
+    const inv = {
+      apply_vat: false,
+      vat_rate: { toNumber: () => 21 },
+      currency: "CZK",
+      invoice_items: [
+        {
+          quantity: { toNumber: () => 2 },
+          unit_price: { toNumber: () => 100 },
+          vat_rate: { toNumber: () => 21 },
+        },
+      ],
+    };
+    expect(invoiceTotalWithVat(inv)).toBe(200);
+  });
+
+  it("rounds each line VAT to 2 decimals before accumulation", () => {
+    // 3 items @ 33.33 with 21% VAT
+    // Line VAT = 33.33 * 0.21 = 6.9993 -> rounded to 7.00
+    // Total VAT = 7.00 * 3 = 21.00
+    // Subtotal = 33.33 * 3 = 99.99
+    // Total = 99.99 + 21.00 = 120.99
+    const inv = {
+      apply_vat: true,
+      vat_rate: { toNumber: () => 21 },
+      currency: "CZK",
+      invoice_items: Array.from({ length: 3 }, () => ({
+        quantity: { toNumber: () => 1 },
+        unit_price: { toNumber: () => 33.33 },
+        vat_rate: { toNumber: () => 21 },
+      })),
+    };
+    expect(invoiceTotalWithVat(inv)).toBe(120.99);
+  });
+
+  it("handles null quantity and unit_price gracefully", () => {
+    const inv = {
+      apply_vat: true,
+      vat_rate: { toNumber: () => 21 },
+      currency: "CZK",
+      invoice_items: [
+        {
+          quantity: { toNumber: () => 0 },
+          unit_price: { toNumber: () => 0 },
+          vat_rate: { toNumber: () => 21 },
+        },
+      ],
+    };
+    expect(invoiceTotalWithVat(inv)).toBe(0);
+  });
+});
diff --git a/src/__tests__/nas-file-manager.test.ts b/src/__tests__/nas-file-manager.test.ts
new file mode 100644
index 0000000..172030e
--- /dev/null
+++ b/src/__tests__/nas-file-manager.test.ts
@@ -0,0 +1,55 @@
+import { describe, it, expect } from "vitest";
+import { NasFileManager } from "../services/nas-file-manager";
+
+describe("NasFileManager path traversal", () => {
+  const nas = new NasFileManager();
+
+  describe("deleteItem", () => {
+    it("rejects empty path", async () => {
+      const result = await nas.deleteItem("PRJ-001", "");
+      expect(result).toContain("kořenovou složku");
+    });
+
+    it("rejects root path /", async () => {
+      const result = await nas.deleteItem("PRJ-001", "/");
+      expect(result).toContain("kořenovou složku");
+    });
+
+    it("rejects current directory .", async () => {
+      const result = await nas.deleteItem("PRJ-001", ".");
+      expect(result).toContain("kořenovou složku");
+    });
+
+    it("rejects current directory ./", async () => {
+      const result = await nas.deleteItem("PRJ-001", "./");
+      expect(result).toContain("kořenovou složku");
+    });
+
+    it("rejects path traversal ..", async () => {
+      const result = await nas.deleteItem("PRJ-001", "../etc/passwd");
+      expect(result).toContain("Neplatná cesta");
+    });
+  });
+
+  describe("moveItem", () => {
+    it("rejects empty fromPath", async () => {
+      const result = await nas.moveItem("PRJ-001", "", "dest");
+      expect(result).toContain("kořenovou složku");
+    });
+
+    it("rejects root fromPath /", async () => {
+      const result = await nas.moveItem("PRJ-001", "/", "dest");
+      expect(result).toContain("kořenovou složku");
+    });
+
+    it("rejects current directory .", async () => {
+      const result = await nas.moveItem("PRJ-001", ".", "dest");
+      expect(result).toContain("kořenovou složku");
+    });
+
+    it("rejects path traversal in fromPath", async () => {
+      const result = await nas.moveItem("PRJ-001", "../secret", "dest");
+      expect(result).toContain("Neplatná cesta");
+    });
+  });
+});
diff --git a/src/__tests__/schema-nan.test.ts b/src/__tests__/schema-nan.test.ts
new file mode 100644
index 0000000..bf496f3
--- /dev/null
+++ b/src/__tests__/schema-nan.test.ts
@@ -0,0 +1,57 @@
+import { describe, it, expect } from "vitest";
+import { CreateOrderSchema } from "../schemas/orders.schema";
+import { CreateQuotationSchema } from "../schemas/offers.schema";
+
+describe("Zod NaN rejection", () => {
+  describe("CreateOrderSchema", () => {
+    it("rejects NaN string in quantity", () => {
+      const result = CreateOrderSchema.safeParse({
+        customer_id: 1,
+        items: [{ quantity: "not-a-number" }],
+      });
+      expect(result.success).toBe(false);
+    });
+
+    it("rejects NaN string in unit_price", () => {
+      const result = CreateOrderSchema.safeParse({
+        customer_id: 1,
+        items: [{ unit_price: "abc" }],
+      });
+      expect(result.success).toBe(false);
+    });
+
+    it("accepts valid numbers", () => {
+      const result = CreateOrderSchema.safeParse({
+        customer_id: 1,
+        items: [{ quantity: 2, unit_price: 100 }],
+      });
+      expect(result.success).toBe(true);
+    });
+  });
+
+  describe("CreateQuotationSchema", () => {
+    it("rejects NaN string in top-level vat_rate", () => {
+      const result = CreateQuotationSchema.safeParse({
+        customer_id: 1,
+        vat_rate: "bad",
+      });
+      expect(result.success).toBe(false);
+    });
+
+    it("accepts valid vat_rate", () => {
+      const result = CreateQuotationSchema.safeParse({
+        customer_id: 1,
+        vat_rate: 21,
+      });
+      expect(result.success).toBe(true);
+    });
+
+    it("rejects NaN string in item quantity", () => {
+      const result = CreateQuotationSchema.safeParse({
+        customer_id: 1,
+        items: [{ quantity: "bad" }],
+      });
+      expect(result.success).toBe(false);
+    });
+  });
+});
diff --git a/src/schemas/offers.schema.ts b/src/schemas/offers.schema.ts
index 79e5e93..d511bf2 100644
--- a/src/schemas/offers.schema.ts
+++ b/src/schemas/offers.schema.ts
@@ -5,14 +5,14 @@ const QuotationItemSchema = z.object({
   item_description: z.string().nullish(),
   quantity: z
     .union([z.number(), z.string()])
-    .transform((v) => Number(v) || 1)
+    .transform((v) => Number(v))
     .refine((v) => !Number.isNaN(v), { message: "Must be a valid number" })
     .optional()
     .default(1),
   unit: z.string().nullish(),
   unit_price: z
     .union([z.number(), z.string()])
-    .transform((v) => Number(v) || 0)
+    .transform((v) => Number(v))
     .refine((v) => !Number.isNaN(v), { message: "Must be a valid number" })
     .optional()
     .default(0),
diff --git a/src/schemas/orders.schema.ts b/src/schemas/orders.schema.ts
index 208e00a..976e956 100644
--- a/src/schemas/orders.schema.ts
+++ b/src/schemas/orders.schema.ts
@@ -5,14 +5,14 @@ const OrderItemSchema = z.object({
   item_description: z.string().nullish(),
   quantity: z
     .union([z.number(), z.string()])
-    .transform((v) => Number(v) || 1)
+    .transform((v) => Number(v))
     .refine((v) => !Number.isNaN(v), { message: "Must be a valid number" })
     .optional()
     .default(1),
   unit: z.string().nullish(),
   unit_price: z
     .union([z.number(), z.string()])
-    .transform((v) => Number(v) || 0)
+    .transform((v) => Number(v))
     .refine((v) => !Number.isNaN(v), { message: "Must be a valid number" })
     .optional()
     .default(0),
diff --git a/src/services/invoices.service.ts b/src/services/invoices.service.ts
index b2824f7..48bfd16 100644
--- a/src/services/invoices.service.ts
+++ b/src/services/invoices.service.ts
@@ -155,7 +155,7 @@ export {
   previewInvoiceNumber as getNextInvoiceNumberPreview,
 } from "./numbering.service";
 
-function invoiceTotalWithVat(inv: {
+export function invoiceTotalWithVat(inv: {
   apply_vat: boolean | null;
   vat_rate: { toNumber(): number } | null;
   currency: string | null;
@@ -166,14 +166,23 @@ function invoiceTotalWithVat(inv: {
   }>;
 }) {
   const sub = inv.invoice_items.reduce(
-    (s, i) => s + (Number(i.quantity) || 0) * (Number(i.unit_price) || 0),
+    (s, i) =>
+      s +
+      (Number(i.quantity?.toNumber()) || 0) *
+        (Number(i.unit_price?.toNumber()) || 0),
     0,
   );
   const vat = inv.apply_vat
     ? inv.invoice_items.reduce((s, i) => {
-        const base = (Number(i.quantity) || 0) * (Number(i.unit_price) || 0);
+        const base =
+          (Number(i.quantity?.toNumber()) || 0) *
+          (Number(i.unit_price?.toNumber()) || 0);
         const lineVat =
-          base * ((Number(i.vat_rate) || Number(inv.vat_rate) || 21) / 100);
+          base *
+          ((Number(i.vat_rate?.toNumber()) ||
+            Number(inv.vat_rate?.toNumber()) ||
+            21) /
+            100);
         return s + Math.round(lineVat * 100) / 100;
       }, 0)
     : 0;