fix: resolve CORS middleware error causing backend restart

The issue was duplicate CORS middleware configuration:
- CORS was configured in main.py (correctly)
- CORS was also configured in security_headers.py (incorrectly)

The security_headers.py version was trying to instantiate CORSMiddleware
directly without the 'app' argument, causing:
TypeError: CORSMiddleware.__init__() missing 1 required positional argument: 'app'

Fixed by:
1. Removed CORS middleware from setup_security_middleware()
2. Updated config.py to include http://localhost:8888 in CORS origins
3. Kept CORS configuration only in main.py

Backend now starts successfully and responds to health checks.

src/core/config.py

@@ -44,8 +44,8 @@ class Settings(BaseSettings):
 
     # Security
     bcrypt_rounds: int = 12
-    cors_allowed_origins: List[str] = ["http://localhost:3000", "http://localhost:5173"]
-    cors_allowed_origins_production: List[str] = []
+    cors_allowed_origins: List[str] = ["http://localhost:3000", "http://localhost:5173", "http://localhost:8888"]
+    cors_allowed_origins_production: List[str] = ["http://localhost:8888"]
 
     # Audit Logging
     audit_logging_enabled: bool = True