chore: upgrade vitest 3.2.4 → 4.0.18

Breaking changes addressed:
- vi.fn() mock factories: arrow → regular functions for constructor support
- Exclude dist/ from test resolution (vitest 4 simplified defaults)
- 672 tests pass, 0 tsc errors

src/__tests__/billing.test.ts

@@ -26,7 +26,7 @@ vi.mock("stripe", () => {
       retrieve: vi.fn(),
     },
   };
-  return { default: vi.fn(() => mockStripe), __mockStripe: mockStripe };
+  return { default: vi.fn(function() { return mockStripe; }), __mockStripe: mockStripe };
 });
 
 let app: express.Express;

src/__tests__/db.test.ts

@@ -6,10 +6,12 @@ const mockQuery = vi.fn();
 const mockConnect = vi.fn();
 
 vi.mock("pg", () => {
-  const Pool = vi.fn(() => ({
-    connect: mockConnect,
-    on: vi.fn(),
-  }));
+  const Pool = vi.fn(function() {
+    return {
+      connect: mockConnect,
+      on: vi.fn(),
+    };
+  });
   return { default: { Pool }, Pool };
 });