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/update-browserslist-db/index.d.ts

@@ -0,0 +1,6 @@
+/**
+ * Run update and print output to terminal.
+ */
+declare function updateDb(print?: (str: string) => void): void
+
+export = updateDb