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/date-fns/locale/da.d.ts

@@ -0,0 +1,12 @@
+import type { Locale } from "./types.js";
+/**
+ * @category Locales
+ * @summary Danish locale.
+ * @language Danish
+ * @iso-639-2 dan
+ * @author Mathias Wøbbe [@MathiasKandelborg](https://github.com/MathiasKandelborg)
+ * @author Anders B. Hansen [@Andersbiha](https://github.com/Andersbiha)
+ * @author [@kgram](https://github.com/kgram)
+ * @author [@stefanbugge](https://github.com/stefanbugge)
+ */
+export declare const da: Locale;