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/models/__init__.py

@@ -5,6 +5,7 @@ from app.models.event_subscription import EventSubscription
 from app.models.invite import InviteStatus, RoomInvite
 from app.models.membership import RoomMembership, RoomRole
 from app.models.message import Message
+from app.models.message_image import MessageImage
 from app.models.push_subscription import PushSubscription
 from app.models.room import Room
 from app.models.user import User
@@ -17,6 +18,7 @@ __all__ = [
     "RoomMembership",
     "RoomRole",
     "Message",
+    "MessageImage",
     "RoomInvite",
     "InviteStatus",
     "PushSubscription",