feat: instance-level streaming limits with admin settings panel

Add a server-side instance_settings table (single-row, CHECK(id=1))
that stores admin-configurable streaming bounds: bitrate min/max/step,
allowed resolutions, and allowed framerates.

Backend:
- New instance_settings schema + migrations (is_admin on users, default
  settings row, first-registered-user promoted to admin)
- GET/PATCH /api/settings/streaming endpoints with admin-only writes
  and full input validation including cross-field checks

Frontend:
- settingsStore fetches limits on WebSocket ready, exposes isAdmin flag
- ScreenShareSettingsPopover reads bounds from store instead of
  hardcoded constants, auto-clamps stale localStorage values
- buildScreenShareOptions() clamps bitrate to server limits at build
  time as enforcement backstop
- ServerSettings modal gains a "Streaming" tab (admin-only) with
  bitrate range sliders, resolution/framerate toggles, and save/reset

packages/server/src/index.ts

@@ -16,6 +16,7 @@ import { uploadRoutes } from './routes/uploads.js';
 import { dmRoutes } from './routes/dm.js';
 import { livekitRoutes } from './routes/livekit.js';
 import { socialRoutes } from './routes/social.js';
+import { settingsRoutes } from './routes/settings.js';
 import { utilRoutes } from './routes/utils.js';
 import { registerWebSocket } from './ws/handler.js';
 import path from 'path';
@@ -72,6 +73,7 @@ async function main(): Promise<void> {
   await app.register(dmRoutes);
   await app.register(livekitRoutes);
   await app.register(socialRoutes);
+  await app.register(settingsRoutes);
   await app.register(utilRoutes);
   await app.register(registerWebSocket);