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/jira_link.py

@@ -2,9 +2,7 @@
 
 import enum
 import uuid
-from datetime import datetime
-
-from sqlalchemy import Column, String, DateTime, ForeignKey, Enum as SQLEnum, Index
+from sqlalchemy import Column, String, DateTime, ForeignKey, Enum as SQLEnum, Index, func
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from sqlalchemy.orm import relationship
 
@@ -45,8 +43,8 @@ class JiraLink(Base):
     last_synced_at = Column(DateTime)
     sync_metadata = Column(JSONB, default={})
     created_by = Column(UUID(as_uuid=True), ForeignKey("users.id"))
-    created_at = Column(DateTime, default=datetime.utcnow)
-    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
 
     creator = relationship("User", foreign_keys=[created_by])