fix(models,db): delegate timestamps to DB server and configure connection pool

- Replace default=datetime.utcnow with server_default=func.now() across all 16 models (17 columns) for consistent, timezone-aware timestamps from PostgreSQL

- Upgrade DateTime columns to DateTime(timezone=True) for timestamptz storage

- Configure SQLAlchemy engine pool: pool_size=20, max_overflow=10, pool_recycle=3600, pool_pre_ping=True

- Remove unused datetime imports from model files

backend/app/database.py

@@ -14,7 +14,13 @@ def _get_engine():
     global _engine
     if _engine is None:
         from app.config import settings
-        _engine = create_engine(settings.DATABASE_URL)
+        _engine = create_engine(
+            settings.DATABASE_URL,
+            pool_size=20,
+            max_overflow=10,
+            pool_recycle=3600,
+            pool_pre_ping=True,
+        )
     return _engine