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.

deploy/backup-postgres.sh

@@ -9,6 +9,10 @@
 #     0 3 * * * /usr/local/bin/chatapp-backup-postgres.sh
 #
 # See ../DEPLOYMENT.md for the full data-server setup this fits into.
+#
+# Covers Postgres only. Uploaded chat images live on the app server's disk
+# (/srv/chatapp/uploads, see app/storage.py), not here -- see DEPLOYMENT.md
+# §7 for that gap.
 
 set -euo pipefail