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/test.py

@@ -1,7 +1,5 @@
 import uuid
-from datetime import datetime
-
-from sqlalchemy import Column, String, Text, Boolean, Integer, DateTime, ForeignKey, Enum
+from sqlalchemy import Column, String, Text, Boolean, Integer, DateTime, ForeignKey, Enum, func
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import relationship
 
@@ -31,7 +29,7 @@ class Test(Base):
     created_by = Column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=True)
     result = Column(Enum(TestResult, name="testresult"), nullable=True)
     state = Column(Enum(TestState, name="teststate"), default=TestState.draft)
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     # ── Red Team fields ─────────────────────────────────────────────
     red_summary = Column(Text, nullable=True)