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/MessageList.css

@@ -51,6 +51,17 @@
   font-family: var(--mono);
 }
 
+.message-image {
+  display: block;
+  max-width: min(320px, 100%);
+  max-height: 240px;
+  object-fit: contain;
+  border-radius: var(--radius);
+  border: 1px solid var(--ds-border);
+  cursor: zoom-in;
+  margin-bottom: 4px;
+}
+
 .message-text {
   font-size: 0.88rem;
   line-height: 1.45;