fix: allow extra environment variables in Settings config

Pydantic Settings now ignores extra env vars like db_user, db_password, redis_password.

Job ID: MTAD-IMPL-2025-11-18-CL

backend/app/config.py

@@ -81,6 +81,7 @@ class Settings(BaseSettings):
     class Config:
         env_file = ".env"
         case_sensitive = False
+        extra = "ignore"
 
 
 settings = Settings()