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/worklog.py

@@ -1,9 +1,7 @@
 """Worklog model — immutable internal time-tracking records."""
 
 import uuid
-from datetime import datetime
-
-from sqlalchemy import Column, String, Integer, DateTime, ForeignKey, Text, Index
+from sqlalchemy import Column, String, Integer, DateTime, ForeignKey, Text, Index, func
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from sqlalchemy.orm import relationship
 
@@ -32,7 +30,7 @@ class Worklog(Base):
     tempo_synced = Column(DateTime)
     tempo_worklog_id = Column(String(100))
     integrity_hash = Column(String(64))
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
     extra_metadata = Column("metadata", JSONB, default={})
 
     user = relationship("User", foreign_keys=[user_id])