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/_lib/buildFormatLongFn.js

@@ -0,0 +1,8 @@
+export function buildFormatLongFn(args) {
+  return (options = {}) => {
+    // TODO: Remove String()
+    const width = options.width ? String(options.width) : args.defaultWidth;
+    const format = args.formats[width] || args.formats[args.defaultWidth];
+    return format;
+  };
+}