Add OpenAPI docs, frontend, migrations, and API updates

- OpenAPI: add missing endpoints (add-from-url, subscriptions, public availability)
- OpenAPI: CalendarSubscription schema, Subscriptions tag
- Frontend app
- Migrations: count_for_availability, subscriptions_sync, user_preferences, calendar_settings
- Config, rate limit, auth, calendar, booking, ICS, availability, user service updates

Made-with: Cursor

frontend/node_modules/@tailwindcss/node/dist/esm-cache.loader.d.mts

@@ -0,0 +1,5 @@
+import { ResolveHook } from 'node:module';
+
+declare let resolve: ResolveHook;
+
+export { resolve };