refactor: move transaction management from services to API endpoints

- Services now use db.flush() instead of db.commit() for database operations
- API endpoints handle transaction commit after service calls
- Remove db.rollback() from services (let exception handlers manage this)
- Ensures consistent transaction boundaries at API layer

This pattern gives API endpoints full control over when to commit,
allowing for better error handling and potential multi-operation transactions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/admin_audit_service.py

@@ -66,7 +66,7 @@ class AdminAuditService:
             )
 
             db.add(audit_log)
-            db.commit()
+            db.flush()
             db.refresh(audit_log)
 
             logger.info(
@@ -77,7 +77,6 @@ class AdminAuditService:
             return audit_log
 
         except Exception as e:
-            db.rollback()
             logger.error(f"Failed to log admin action: {str(e)}")
             # Don't raise exception - audit logging should not break operations
             return None