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/isSaturday.js

@@ -0,0 +1,30 @@
+import { toDate } from "./toDate.js";
+
+/**
+ * The {@link isSaturday} function options.
+ */
+
+/**
+ * @name isSaturday
+ * @category Weekday Helpers
+ * @summary Is the given date Saturday?
+ *
+ * @description
+ * Is the given date Saturday?
+ *
+ * @param date - The date to check
+ * @param options - An object with options
+ *
+ * @returns The date is Saturday
+ *
+ * @example
+ * // Is 27 September 2014 Saturday?
+ * const result = isSaturday(new Date(2014, 8, 27))
+ * //=> true
+ */
+export function isSaturday(date, options) {
+  return toDate(date, options?.in).getDay() === 6;
+}
+
+// Fallback for modularized imports:
+export default isSaturday;