Add comprehensive test framework with vitest and TDD tests

- Set up vitest test framework with proper configuration
- Added test scripts to package.json (test, test:watch, test:ui)
- Created comprehensive SSRF validation tests (30 tests)
  - Tests for protocol validation (HTTP/HTTPS only)
  - Private IP blocking (127.x, 10.x, 172.16-31.x, 192.168.x, 169.254.x)
  - Kubernetes service DNS blocking (.svc, .cluster.local, etc.)
  - URL length validation (max 2048 chars)
  - DNS resolution error handling
  - Edge cases with ports, query params, userinfo
- Created cache service tests (19 tests)
  - Cache hit/miss operations
  - Deterministic key generation
  - TTL expiry behavior
  - Size limits and LRU eviction
  - Cache bypass logic
  - Statistics tracking
- Created integration test suite (marked as skip for CI)
  - Health endpoint tests
  - Playground endpoint tests with rate limiting
  - Authentication tests for screenshot endpoints
  - CORS header validation
  - Error handling and security headers
- All unit tests pass (49 total tests)
- Following strict Red/Green TDD methodology

package.json

@@ -7,7 +7,10 @@
   "scripts": {
     "build": "tsc",
     "start": "node dist/index.js",
-    "dev": "tsx src/index.ts"
+    "dev": "tsx src/index.ts",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "test:ui": "vitest --ui"
   },
   "dependencies": {
     "compression": "^1.8.1",
@@ -26,8 +29,12 @@
     "@types/express": "^5.0.0",
     "@types/node": "^22.0.0",
     "@types/pg": "^8.0.0",
+    "@types/supertest": "^6.0.3",
     "@types/swagger-jsdoc": "^6.0.4",
+    "@vitest/ui": "^4.0.18",
+    "supertest": "^7.2.2",
     "tsx": "^4.0.0",
-    "typescript": "^5.6.0"
+    "typescript": "^5.6.0",
+    "vitest": "^4.0.18"
   }
 }