feat(ai): spend tracker + AI service (chat, extractInvoice)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/__tests__/ai.test.ts

@@ -0,0 +1,93 @@
+import { describe, it, expect, beforeEach, afterAll } from "vitest";
+import prisma from "../config/database";
+import {
+  computeCostUsd,
+  recordUsage,
+  getMonthSpendUsd,
+  getBudgetUsd,
+  assertBudgetAvailable,
+  isConfigured,
+} from "../services/ai.service";
+
+const KIND = "test_ai"; // marker so we only clean our own rows
+
+beforeEach(async () => {
+  await prisma.ai_usage.deleteMany({ where: { kind: KIND } });
+});
+afterAll(async () => {
+  await prisma.ai_usage.deleteMany({ where: { kind: KIND } });
+});
+
+describe("ai.service cost + budget", () => {
+  it("computes Sonnet 4.6 cost from tokens", () => {
+    // 1,000,000 input @ $3 + 1,000,000 output @ $15 = $18
+    expect(
+      computeCostUsd("claude-sonnet-4-6", 1_000_000, 1_000_000),
+    ).toBeCloseTo(18, 6);
+    expect(computeCostUsd("claude-sonnet-4-6", 4000, 500)).toBeCloseTo(
+      0.0195,
+      6,
+    );
+  });
+
+  it("records usage with the computed cost", async () => {
+    await recordUsage({
+      userId: null,
+      kind: KIND,
+      model: "claude-sonnet-4-6",
+      inputTokens: 4000,
+      outputTokens: 500,
+    });
+    const rows = await prisma.ai_usage.findMany({ where: { kind: KIND } });
+    expect(rows.length).toBe(1);
+    expect(Number(rows[0].cost_usd)).toBeCloseTo(0.0195, 6);
+  });
+
+  it("sums only the current month's spend", async () => {
+    await recordUsage({
+      userId: null,
+      kind: KIND,
+      model: "claude-sonnet-4-6",
+      inputTokens: 1_000_000,
+      outputTokens: 0,
+    }); // $3 this month
+    // An old row (last year) must NOT count.
+    await prisma.ai_usage.create({
+      data: {
+        kind: KIND,
+        model: "claude-sonnet-4-6",
+        input_tokens: 1_000_000,
+        output_tokens: 0,
+        cost_usd: 3,
+        created_at: new Date("2000-01-01T00:00:00Z"),
+      },
+    });
+    const spend = await getMonthSpendUsd();
+    expect(spend).toBeGreaterThanOrEqual(3);
+    expect(spend).toBeLessThan(6); // the year-2000 $3 is excluded
+  });
+
+  it("assertBudgetAvailable blocks at/over budget, allows under", async () => {
+    const budget = await getBudgetUsd();
+    // Under budget → null (allowed)
+    expect(await assertBudgetAvailable()).toBeNull();
+    // Push spend over budget for this month, then it must block with 402.
+    await prisma.ai_usage.create({
+      data: {
+        kind: KIND,
+        model: "claude-sonnet-4-6",
+        input_tokens: 0,
+        output_tokens: 0,
+        cost_usd: budget + 1,
+        created_at: new Date(),
+      },
+    });
+    const blocked = await assertBudgetAvailable();
+    expect(blocked).not.toBeNull();
+    expect(blocked?.status).toBe(402);
+  });
+
+  it("isConfigured reflects the API key presence", () => {
+    expect(typeof isConfigured()).toBe("boolean");
+  });
+});