test: add comprehensive unit tests for core services

Added unit tests for the `api-keys`, `auth`, `categories`, `contents`, `favorites`, `media`, and `purge` services to improve test coverage and ensure core functionality integrity.

backend/src/health.controller.spec.ts

@@ -0,0 +1,42 @@
+import { Test, TestingModule } from "@nestjs/testing";
+import { DatabaseService } from "./database/database.service";
+import { HealthController } from "./health.controller";
+
+describe("HealthController", () => {
+	let controller: HealthController;
+
+	const mockDb = {
+		execute: jest.fn().mockResolvedValue([]),
+	};
+
+	beforeEach(async () => {
+		const module: TestingModule = await Test.createTestingModule({
+			controllers: [HealthController],
+			providers: [
+				{
+					provide: DatabaseService,
+					useValue: {
+						db: mockDb,
+					},
+				},
+			],
+		}).compile();
+
+		controller = module.get<HealthController>(HealthController);
+	});
+
+	it("should return ok if database is connected", async () => {
+		mockDb.execute.mockResolvedValue([]);
+		const result = await controller.check();
+		expect(result.status).toBe("ok");
+		expect(result.database).toBe("connected");
+	});
+
+	it("should return error if database is disconnected", async () => {
+		mockDb.execute.mockRejectedValue(new Error("DB Error"));
+		const result = await controller.check();
+		expect(result.status).toBe("error");
+		expect(result.database).toBe("disconnected");
+		expect(result.message).toBe("DB Error");
+	});
+});