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/isLeapYear.d.cts

@@ -0,0 +1,24 @@
+import type { ContextOptions, DateArg } from "./types.js";
+export interface IsLeapYearOptions extends ContextOptions<Date> {}
+/**
+ * @name isLeapYear
+ * @category Year Helpers
+ * @summary Is the given date in the leap year?
+ *
+ * @description
+ * Is the given date in the leap year?
+ *
+ * @param date - The date to check
+ * @param options - The options object
+ *
+ * @returns The date is in the leap year
+ *
+ * @example
+ * // Is 1 September 2012 in the leap year?
+ * const result = isLeapYear(new Date(2012, 8, 1))
+ * //=> true
+ */
+export declare function isLeapYear(
+  date: DateArg<Date> & {},
+  options?: IsLeapYearOptions | undefined,
+): boolean;