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/coverage_snapshot.py

@@ -6,11 +6,9 @@ per technique per snapshot) to avoid bloated JSONB fields.
 """
 
 import uuid
-from datetime import datetime
-
 from sqlalchemy import (
     Column, String, Float, Integer, DateTime,
-    ForeignKey, Index,
+    ForeignKey, Index, func,
 )
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import relationship
@@ -37,7 +35,7 @@ class CoverageSnapshot(Base):
         ForeignKey("users.id", ondelete="SET NULL"),
         nullable=True,
     )
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     # Relationships
     creator = relationship("User", foreign_keys=[created_by])