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/audit.py

@@ -1,7 +1,5 @@
 import uuid
-from datetime import datetime
-
-from sqlalchemy import Column, String, DateTime, ForeignKey
+from sqlalchemy import Column, String, DateTime, ForeignKey, func
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from sqlalchemy.orm import relationship
 
@@ -22,7 +20,7 @@ class AuditLog(Base):
     action = Column(String, nullable=False)
     entity_type = Column(String, nullable=True)
     entity_id = Column(String, nullable=True)
-    timestamp = Column(DateTime, default=datetime.utcnow)
+    timestamp = Column(DateTime(timezone=True), server_default=func.now())
     details = Column(JSONB, nullable=True)
 
     # Relationships