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/schemas/message.py

@@ -11,6 +11,7 @@ class MessageRead(BaseModel):
     room_id: uuid.UUID
     user_id: uuid.UUID
     username: str
-    content: str
+    content: str | None
+    image_id: uuid.UUID | None
     created_at: datetime
     edited_at: datetime | None

backend/app/schemas/message_image.py

@@ -0,0 +1,7 @@
+import uuid
+
+from pydantic import BaseModel
+
+
+class MessageImageCreated(BaseModel):
+    id: uuid.UUID