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/pyproject.toml

@@ -18,6 +18,8 @@ dependencies = [
     "redis>=5.0",
     "httpx>=0.27",
     "gunicorn>=23.0",
+    "Pillow>=10.0",
+    "python-multipart>=0.0.9",
 ]
 
 [project.scripts]