Migrate from Express 4 to Express 5

- Upgraded express from ^4.22.1 to ^5.2.1
- Added comprehensive Express 5 migration tests with TDD approach
- All 667 tests passing (663 existing + 4 new migration tests)
- No breaking changes detected in the codebase
- Express 5's native async error handling now active
- TypeScript compilation successful with @types/express ^5.0.6

Express 5 features now available:
- Automatic async error catching in route handlers
- Improved performance and stricter path matching
- Default export import style already in use

package.json

@@ -13,7 +13,7 @@
   },
   "dependencies": {
     "compression": "^1.8.1",
-    "express": "^4.22.1",
+    "express": "^5.1.0",
     "express-rate-limit": "^7.5.1",
     "helmet": "^8.1.0",
     "marked": "^17.0.4",