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/threat_actor.py

@@ -5,11 +5,9 @@ techniques, imported from MITRE CTI (STIX 2.0).
 """
 
 import uuid
-from datetime import datetime
-
 from sqlalchemy import (
     Column, String, Text, Boolean, DateTime,
-    ForeignKey, Index, UniqueConstraint,
+    ForeignKey, Index, UniqueConstraint, func,
 )
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from sqlalchemy.orm import relationship
@@ -40,7 +38,7 @@ class ThreatActor(Base):
     references = Column(JSONB, nullable=True, default=[])               # [{"url": "...", "description": "..."}]
     mitre_url = Column(String, nullable=True)
     is_active = Column(Boolean, default=True)
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     # Relationships
     techniques = relationship(