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/en-GB.d.cts

@@ -0,0 +1,9 @@
+import type { Locale } from "./types.js";
+/**
+ * @category Locales
+ * @summary English locale (United Kingdom).
+ * @language English
+ * @iso-639-2 eng
+ * @author Alex [@glintik](https://github.com/glintik)
+ */
+export declare const enGB: Locale;