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/nl.d.cts

@@ -0,0 +1,14 @@
+import type { Locale } from "./types.js";
+/**
+ * @category Locales
+ * @summary Dutch locale.
+ * @language Dutch
+ * @iso-639-2 nld
+ * @author Jorik Tangelder [@jtangelder](https://github.com/jtangelder)
+ * @author Ruben Stolk [@rubenstolk](https://github.com/rubenstolk)
+ * @author Lode Vanhove [@bitcrumb](https://github.com/bitcrumb)
+ * @author Edo Rivai [@edorivai](https://github.com/edorivai)
+ * @author Niels Keurentjes [@curry684](https://github.com/curry684)
+ * @author Stefan Vermaas [@stefanvermaas](https://github.com/stefanvermaas)
+ */
+export declare const nl: Locale;