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/de.d.cts

@@ -0,0 +1,13 @@
+import type { Locale } from "./types.js";
+/**
+ * @category Locales
+ * @summary German locale.
+ * @language German
+ * @iso-639-2 deu
+ * @author Thomas Eilmsteiner [@DeMuu](https://github.com/DeMuu)
+ * @author Asia [@asia-t](https://github.com/asia-t)
+ * @author Van Vuong Ngo [@vanvuongngo](https://github.com/vanvuongngo)
+ * @author RomanErnst [@pex](https://github.com/pex)
+ * @author Philipp Keck [@Philipp91](https://github.com/Philipp91)
+ */
+export declare const de: Locale;