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/defensive_technique.py

@@ -5,11 +5,9 @@ ATT&CK techniques, enabling recommended countermeasure lookups.
 """
 
 import uuid
-from datetime import datetime
-
 from sqlalchemy import (
     Column, String, Text, DateTime,
-    ForeignKey, Index, UniqueConstraint,
+    ForeignKey, Index, UniqueConstraint, func,
 )
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import relationship
@@ -32,7 +30,7 @@ class DefensiveTechnique(Base):
     description = Column(Text, nullable=True)
     tactic = Column(String, nullable=True)                            # Detect, Isolate, Deceive, Evict, etc.
     d3fend_url = Column(String, nullable=True)
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     # Relationships
     attack_mappings = relationship(