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/api/v1/admin/vendor_themes.py

@@ -136,6 +136,7 @@ async def update_vendor_theme(
     # Service handles all validation and raises appropriate exceptions
     # Global exception handler converts them to proper HTTP responses
     theme = vendor_theme_service.update_theme(db, vendor_code, theme_data)
+    db.commit()
     return VendorThemeResponse(**theme.to_dict())
 
 
@@ -185,6 +186,7 @@ async def apply_theme_preset(
     # Raises ThemePresetNotFoundException if preset doesn't exist
     # Global exception handler converts to HTTP 404
     theme = vendor_theme_service.apply_theme_preset(db, vendor_code, preset_name)
+    db.commit()
 
     return ThemePresetResponse(
         message=f"Applied {preset_name} preset successfully",
@@ -226,4 +228,5 @@ async def delete_vendor_theme(
     # Service handles deletion and raises exceptions if needed
     # Global exception handler converts them to proper HTTP responses
     result = vendor_theme_service.delete_theme(db, vendor_code)
+    db.commit()
     return ThemeDeleteResponse(message=result.get("message", "Theme deleted successfully"))