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/formatISODuration.d.cts

@@ -0,0 +1,26 @@
+import type { Duration } from "./types.js";
+/**
+ * @name formatISODuration
+ * @category Common Helpers
+ * @summary Format a duration object according as ISO 8601 duration string
+ *
+ * @description
+ * Format a duration object according to the ISO 8601 duration standard (https://www.digi.com/resources/documentation/digidocs//90001488-13/reference/r_iso_8601_duration_format.htm)
+ *
+ * @param duration - The duration to format
+ *
+ * @returns The ISO 8601 duration string
+ *
+ * @example
+ * // Format the given duration as ISO 8601 string
+ * const result = formatISODuration({
+ *   years: 39,
+ *   months: 2,
+ *   days: 20,
+ *   hours: 7,
+ *   minutes: 5,
+ *   seconds: 0
+ * })
+ * //=> 'P39Y2M20DT0H0M0S'
+ */
+export declare function formatISODuration(duration: Duration): string;