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/notification.py

@@ -1,9 +1,7 @@
 """Notification model — in-app notifications for user actions."""
 
 import uuid
-from datetime import datetime
-
-from sqlalchemy import Column, String, Text, Boolean, DateTime, ForeignKey, Index
+from sqlalchemy import Column, String, Text, Boolean, DateTime, ForeignKey, Index, func
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import relationship
 
@@ -27,7 +25,7 @@ class Notification(Base):
     entity_type = Column(String, nullable=True)
     entity_id = Column(UUID(as_uuid=True), nullable=True)
     read = Column(Boolean, default=False)
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     # Relationships
     user = relationship("User")