fix(backend): delete_attachment 204 response must not declare a body

Litestar validates that handlers with 204 status codes cannot return
a response body. The delete_attachment handler was declared as
-> Response and returned Response(content=None, status_code=204),
which caused ImproperlyConfiguredException at startup and crashed
all workers. Change return type to None and move status_code to the
@delete decorator.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/attachments.py

@@ -181,8 +181,8 @@ async def download_attachment(attachment_id: str) -> Response:
     )
 
 
-@delete("/api/attachments/{attachment_id:str}")
-async def delete_attachment(attachment_id: str) -> Response:
+@delete("/api/attachments/{attachment_id:str}", status_code=204)
+async def delete_attachment(attachment_id: str) -> None:
     """DELETE /api/attachments/:id -- Delete attachment (blob + DB record)."""
     async with async_session_factory() as session:
         attachment = await session.get(Attachment, attachment_id)
@@ -211,5 +211,3 @@ async def delete_attachment(attachment_id: str) -> Response:
         # Delete from database
         await session.delete(attachment)
         await session.commit()
-
-    return Response(content=None, status_code=204)