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/formatRFC7231.d.cts

@@ -0,0 +1,22 @@
+import type { DateArg } from "./types.js";
+/**
+ * @name formatRFC7231
+ * @category Common Helpers
+ * @summary Format the date according to the RFC 7231 standard (https://tools.ietf.org/html/rfc7231#section-7.1.1.1).
+ *
+ * @description
+ * Return the formatted date string in RFC 7231 format.
+ * The result will always be in UTC timezone.
+ *
+ * @param date - The original date
+ *
+ * @returns The formatted date string
+ *
+ * @throws `date` must not be Invalid Date
+ *
+ * @example
+ * // Represent 18 September 2019 in RFC 7231 format:
+ * const result = formatRFC7231(new Date(2019, 8, 18, 19, 0, 52))
+ * //=> 'Wed, 18 Sep 2019 19:00:52 GMT'
+ */
+export declare function formatRFC7231(date: DateArg<Date> & {}): string;