feat(#3): Add comprehensive tests and improve Prisma seed script

- Create comprehensive test suite for PrismaService (10 tests)
- Fix AppController tests with proper PrismaService mocking
- Wrap seed operations in transaction for atomicity
- Replace N+1 pattern with batch operations (createMany)
- Add concurrency warning to seed script
- All tests passing (14/14)
- Build successful
- Test coverage >85%

Fixes #3

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/src/app.controller.test.ts

@@ -1,10 +1,20 @@
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi } from "vitest";
 import { AppService } from "./app.service";
 import { AppController } from "./app.controller";
+import { PrismaService } from "./prisma/prisma.service";
 
 describe("AppController", () => {
   const appService = new AppService();
-  const controller = new AppController(appService);
+
+  // Mock PrismaService
+  const mockPrismaService = {
+    isHealthy: vi.fn(),
+    getConnectionInfo: vi.fn(),
+    $connect: vi.fn(),
+    $disconnect: vi.fn(),
+  } as unknown as PrismaService;
+
+  const controller = new AppController(appService, mockPrismaService);
 
   describe("getHello", () => {
     it('should return "Mosaic Stack API"', () => {
@@ -13,11 +23,33 @@ describe("AppController", () => {
   });
 
   describe("getHealth", () => {
-    it("should return health status", () => {
-      const result = controller.getHealth();
+    it("should return health status", async () => {
+      // Setup mocks
+      vi.mocked(mockPrismaService.isHealthy).mockResolvedValue(true);
+      vi.mocked(mockPrismaService.getConnectionInfo).mockResolvedValue({
+        connected: true,
+        database: "mosaic",
+        version: "PostgreSQL 17",
+      });
+
+      const result = await controller.getHealth();
       expect(result.success).toBe(true);
       expect(result.data.status).toBe("healthy");
       expect(result.data.timestamp).toBeDefined();
+      expect(result.data.checks.database.status).toBe("healthy");
+    });
+
+    it("should return degraded status when database is unhealthy", async () => {
+      // Setup mocks for unhealthy state
+      vi.mocked(mockPrismaService.isHealthy).mockResolvedValue(false);
+      vi.mocked(mockPrismaService.getConnectionInfo).mockResolvedValue({
+        connected: false,
+      });
+
+      const result = await controller.getHealth();
+      expect(result.success).toBe(true);
+      expect(result.data.status).toBe("degraded");
+      expect(result.data.checks.database.status).toBe("unhealthy");
     });
   });
 });