test: add regression tests for Critical+High FLAWS_REPORT fixes

- Tests caught 2 real bugs:
  - Zod NaN bypass in orders/offers schemas (Number(v) || fallback)
  - invoiceTotalWithVat using Number() on { toNumber() } objects
- 7 new test files covering auth, env, exchange rates, NAS paths,
  schema NaN rejection, invoice VAT calculation, customer validation
- 45 tests passing, build clean

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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();
+  });
+});