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/_lib/defaultOptions.d.cts

@@ -0,0 +1,11 @@
+import type {
+  FirstWeekContainsDateOptions,
+  Locale,
+  LocalizedOptions,
+  WeekOptions,
+} from "../types.js";
+export type DefaultOptions = LocalizedOptions<keyof Locale> &
+  WeekOptions &
+  FirstWeekContainsDateOptions;
+export declare function getDefaultOptions(): DefaultOptions;
+export declare function setDefaultOptions(newOptions: DefaultOptions): void;