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/data_source.py

@@ -1,9 +1,7 @@
 """DataSource model — registry of external data sources for import."""
 
 import uuid
-from datetime import datetime
-
-from sqlalchemy import Column, String, Text, Boolean, DateTime, Index
+from sqlalchemy import Column, String, Text, Boolean, DateTime, Index, func
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 
 from app.database import Base
@@ -31,7 +29,7 @@ class DataSource(Base):
     last_sync_stats = Column(JSONB, nullable=True)                 # {"imported": X, "updated": Y, ...}
     sync_frequency = Column(String, nullable=True)                 # daily / weekly / monthly / manual
     config = Column(JSONB, nullable=True)                          # source-specific configuration
-    created_at = Column(DateTime, default=datetime.utcnow)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     __table_args__ = (
         Index('ix_data_sources_type', 'type'),