Add admin-configurable upload size limits

The 8MB image/file/avatar cap is now a site setting (UploadSettings,
single-row table like SmtpSettings) editable from the Admin Settings tab,
instead of a hardcoded constant. All three upload endpoints read the live
value and interpolate it into their 413 messages. A new GET
/api/uploads/limit endpoint (open to any authenticated user, unlike the
admin-only settings endpoints) lets the composer reject an oversized file
client-side before it ever hits the network, though the server still
enforces the same cap independently.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/main.py

@@ -11,7 +11,7 @@ from redis.asyncio import Redis
 from starlette.middleware.sessions import SessionMiddleware
 
 from app.config import settings
-from app.routers import admin, auth, bots, health, push, rooms, signup, users, webhooks
+from app.routers import admin, auth, bots, health, push, rooms, signup, uploads, users, webhooks
 from app.ws.broadcaster import RoomBroadcaster
 from app.ws.chat import router as ws_router
 from app.ws.connection_manager import ConnectionManager
@@ -76,6 +76,7 @@ def create_app() -> FastAPI:
     app.include_router(rooms.router)
     app.include_router(users.router)
     app.include_router(push.router)
+    app.include_router(uploads.router)
     app.include_router(admin.router)
     app.include_router(bots.router)
     app.include_router(webhooks.router)