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/nanoid/url-alphabet/package.json

@@ -0,0 +1,6 @@
+{
+  "type": "module",
+  "main": "index.cjs",
+  "module": "index.js",
+  "react-native": "index.js"
+}