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.cjs

@@ -0,0 +1,27 @@
+"use strict";
+exports.isLeapYear = isLeapYear;
+var _index = require("./toDate.cjs");
+
+/**
+ * @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
+ */
+function isLeapYear(date, options) {
+  const _date = (0, _index.toDate)(date, options?.in);
+  const year = _date.getFullYear();
+  return year % 400 === 0 || (year % 4 === 0 && year % 100 !== 0);
+}