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.

frontend/src/components/ChatPane.tsx

@@ -99,8 +99,8 @@ export function ChatPane({ room, members, isMobile, onBack, onToggleInfo, infoOp
         </p>
       )}
 
-      <MessageList messages={[...history, ...live]} members={members} onEdit={sendEdit} />
-      <Composer roomName={room.name} disabled={!connected} onSend={send} />
+      <MessageList roomId={room.id} messages={[...history, ...live]} members={members} onEdit={sendEdit} />
+      <Composer roomId={room.id} roomName={room.name} disabled={!connected} onSend={send} />
     </section>
   )
 }