Add image uploads in chat messages (Gitea issue #10)

Images live on the app server's local disk (uploads/), served through an
authenticated, room-membership-gated endpoint since rooms can be private.
Uploads are streamed with a byte-count cap, validated as genuine decodable
images with Pillow (not just a spoofed Content-Type), and downscaled to
2000px on the longer side (except GIF, to preserve animation).

Backend: MessageImage model + nullable Message.content/image_id with a
content-or-image CheckConstraint, upload/serve endpoints in rooms.py, WS
message envelope gains image_id, push notification body says "sent an
image" for image-only messages.

Frontend: Composer gets an attach button with upload progress and a
thumbnail chip; MessageList renders images inline with a click-to-zoom
ImageLightbox.

backend/app/services/message_service.py

@@ -17,9 +17,13 @@ class NotMessageAuthorError(Exception):
 
 
 async def create_message(
-    db: AsyncSession, room_id: uuid.UUID, user_id: uuid.UUID, content: str
+    db: AsyncSession,
+    room_id: uuid.UUID,
+    user_id: uuid.UUID,
+    content: str | None = None,
+    image_id: uuid.UUID | None = None,
 ) -> Message:
-    message = Message(room_id=room_id, user_id=user_id, content=content)
+    message = Message(room_id=room_id, user_id=user_id, content=content, image_id=image_id)
     db.add(message)
     await db.commit()
     await db.refresh(message)