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/intervalToDuration.d.cts

@@ -0,0 +1,30 @@
+import type { ContextOptions, Duration, Interval } from "./types.js";
+/**
+ * The {@link intervalToDuration} function options.
+ */
+export interface IntervalToDurationOptions extends ContextOptions<Date> {}
+/**
+ * @name intervalToDuration
+ * @category Common Helpers
+ * @summary Convert interval to duration
+ *
+ * @description
+ * Convert an interval object to a duration object.
+ *
+ * @param interval - The interval to convert to duration
+ * @param options - The context options
+ *
+ * @returns The duration object
+ *
+ * @example
+ * // Get the duration between January 15, 1929 and April 4, 1968.
+ * intervalToDuration({
+ *   start: new Date(1929, 0, 15, 12, 0, 0),
+ *   end: new Date(1968, 3, 4, 19, 5, 0)
+ * });
+ * //=> { years: 39, months: 2, days: 20, hours: 7, minutes: 5, seconds: 0 }
+ */
+export declare function intervalToDuration(
+  interval: Interval,
+  options?: IntervalToDurationOptions | undefined,
+): Duration;