test: add unit and integration test suites

- Extract evaluateCondition + rule types into src/lib/rules.ts for testability
- 48 unit tests for evaluateCondition (all fields/operators) and formatCategory
- 21 integration tests for getTransactions filters and getParticipantBalances
- Vitest configs for unit (vitest.config.ts) and integration (vitest.integration.config.ts)
- setup-test-db.sh creates personal_test DB from production schema via pg_dump
- Use vi.doMock + dynamic import pattern to isolate test DB from Prisma singleton

src/__tests__/integration/participants.test.ts

@@ -0,0 +1,76 @@
+import { describe, it, expect, beforeEach, afterAll } from "vitest";
+import { createPool, resetDB } from "./helpers";
+
+const pool = createPool();
+
+beforeEach(async () => {
+  await resetDB(pool);
+});
+
+afterAll(async () => {
+  await pool.end();
+});
+
+// This tests the name-substitution logic applied in /api/participants GET.
+// The rule: the participant matching the current user's ID gets name "Me";
+// everyone else keeps their real name.
+function substituteMe(
+  participants: { id: number; name: string }[],
+  currentUserId: number
+) {
+  return participants.map((p) =>
+    p.id === currentUserId ? { ...p, name: "Me" } : p
+  );
+}
+
+describe("participant Me substitution", () => {
+  it("replaces the current user's name with Me", async () => {
+    const r = await pool.query(
+      `INSERT INTO participants (name) VALUES ('Siddharth') RETURNING id`
+    );
+    const userId = r.rows[0].id;
+    const participants = [
+      { id: userId, name: "Siddharth" },
+      { id: userId + 1, name: "Sonu" },
+    ];
+
+    const result = substituteMe(participants, userId);
+    expect(result.find((p) => p.id === userId)?.name).toBe("Me");
+    expect(result.find((p) => p.id === userId + 1)?.name).toBe("Sonu");
+  });
+
+  it("leaves all names unchanged when currentUserId does not match", () => {
+    const participants = [
+      { id: 1, name: "Siddharth" },
+      { id: 2, name: "Sonu" },
+    ];
+    const result = substituteMe(participants, 999);
+    expect(result).toEqual(participants);
+  });
+
+  it("Sonu sees Me for herself and Siddharth for the primary user", async () => {
+    const r1 = await pool.query(
+      `INSERT INTO participants (name) VALUES ('Siddharth') RETURNING id`
+    );
+    const r2 = await pool.query(
+      `INSERT INTO participants (name, email) VALUES ('Sonu', 'sonu@example.com') RETURNING id`
+    );
+    const siddharthId = r1.rows[0].id;
+    const sonuId = r2.rows[0].id;
+
+    const rawParticipants = [
+      { id: siddharthId, name: "Siddharth" },
+      { id: sonuId, name: "Sonu" },
+    ];
+
+    // Siddharth's view
+    const siddharthView = substituteMe(rawParticipants, siddharthId);
+    expect(siddharthView.find((p) => p.id === siddharthId)?.name).toBe("Me");
+    expect(siddharthView.find((p) => p.id === sonuId)?.name).toBe("Sonu");
+
+    // Sonu's view
+    const sonuView = substituteMe(rawParticipants, sonuId);
+    expect(sonuView.find((p) => p.id === siddharthId)?.name).toBe("Siddharth");
+    expect(sonuView.find((p) => p.id === sonuId)?.name).toBe("Me");
+  });
+});