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_settings_service.py

@@ -163,7 +163,7 @@ class AdminSettingsService:
             )
 
             db.add(setting)
-            db.commit()
+            db.flush()
             db.refresh(setting)
 
             logger.info(f"Setting '{setting.key}' created by admin {admin_user_id}")
@@ -171,10 +171,8 @@ class AdminSettingsService:
             return AdminSettingResponse.model_validate(setting)
 
         except ValidationException:
-            db.rollback()
             raise
         except Exception as e:
-            db.rollback()
             logger.error(f"Failed to create setting: {str(e)}")
             raise AdminOperationException(
                 operation="create_setting", reason="Database operation failed"
@@ -205,7 +203,7 @@ class AdminSettingsService:
             setting.last_modified_by_user_id = admin_user_id
             setting.updated_at = datetime.now(UTC)
 
-            db.commit()
+            db.flush()
             db.refresh(setting)
 
             logger.info(f"Setting '{setting.key}' updated by admin {admin_user_id}")
@@ -213,10 +211,8 @@ class AdminSettingsService:
             return AdminSettingResponse.model_validate(setting)
 
         except ValidationException:
-            db.rollback()
             raise
         except Exception as e:
-            db.rollback()
             logger.error(f"Failed to update setting {key}: {str(e)}")
             raise AdminOperationException(
                 operation="update_setting", reason="Database operation failed"
@@ -244,14 +240,12 @@ class AdminSettingsService:
 
         try:
             db.delete(setting)
-            db.commit()
 
             logger.warning(f"Setting '{key}' deleted by admin {admin_user_id}")
 
             return f"Setting '{key}' successfully deleted"
 
         except Exception as e:
-            db.rollback()
             logger.error(f"Failed to delete setting {key}: {str(e)}")
             raise AdminOperationException(
                 operation="delete_setting", reason="Database operation failed"