feat(m5b): optional YouTube write scope via incremental OAuth

Default login now requests read-only (youtube.readonly); write (unsubscribe,
later playlist export) is an explicit opt-in.

- auth.py: split READ_SCOPES / WRITE_SCOPES; new GET /auth/upgrade (incremental
  consent, prompt=consent); has_write_scope() helper
- /api/me exposes can_write
- youtube/client.py: delete_subscription (50 units, OAuth-only)
- DELETE /api/channels/{id}/subscription, gated on write scope (403 otherwise)
- UI: Settings - Account 'Playlist editing & YouTube export' enable button;
  per-channel 'Unsubscribe on YouTube' (with confirm) shown only when can_write

Browser-facing; develop/test locally until the public HTTPS login lands. Needs a
one-time Console step: add youtube.readonly to the OAuth consent screen scopes.

frontend/src/App.tsx

@@ -152,6 +152,7 @@ export default function App() {
             <Feed filters={filters} setFilters={setFilters} view={view} />
           ) : (
             <Channels
+              canWrite={meQuery.data!.can_write}
               onViewChannel={(id, name) => {
                 setFilters({ ...filters, channelId: id, channelName: name, show: "all" });
                 setPage("feed");