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
