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/osint_item.py

@@ -1,9 +1,7 @@
 """OSINT enrichment items — CVEs, blogs, PoCs, and advisories linked to techniques."""
 
 import uuid
-from datetime import datetime
-
-from sqlalchemy import Column, String, Text, Boolean, DateTime, ForeignKey
+from sqlalchemy import Column, String, Text, Boolean, DateTime, ForeignKey, func
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from sqlalchemy.orm import relationship
 
@@ -32,7 +30,7 @@ class OsintItem(Base):
     title = Column(String(500), nullable=False)
     description = Column(Text, nullable=True)
     severity = Column(String(20), nullable=True)  # CRITICAL, HIGH, MEDIUM, LOW, UNKNOWN
-    discovered_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+    discovered_at = Column(DateTime(timezone=True), server_default=func.now(), nullable=False)
     reviewed = Column(Boolean, default=False)
     metadata_ = Column("metadata", JSONB, default={})