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/models/user.py

@@ -1,7 +1,5 @@
 import uuid
-from datetime import datetime
-
-from sqlalchemy import Column, String, Boolean, DateTime
+from sqlalchemy import Column, String, Boolean, DateTime, func
 from sqlalchemy.dialects.postgresql import UUID
 
 from app.database import Base
@@ -28,5 +26,5 @@ class User(Base):
     role = Column(String, nullable=False, default="viewer")
     is_active = Column(Boolean, default=True)
     must_change_password = Column(Boolean, default=True)
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
     last_login = Column(DateTime, nullable=True)