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/browserslist/error.d.ts

@@ -0,0 +1,7 @@
+declare class BrowserslistError extends Error {
+  constructor(message: any)
+  name: 'BrowserslistError'
+  browserslist: true
+}
+
+export = BrowserslistError