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.d.ts

@@ -0,0 +1,27 @@
+import type { ContextOptions, DateArg } from "./types.js";
+/**
+ * The {@link isSaturday} function options.
+ */
+export interface IsSaturdayOptions extends ContextOptions<Date> {}
+/**
+ * @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 declare function isSaturday(
+  date: DateArg<Date> & {},
+  options?: IsSaturdayOptions | undefined,
+): boolean;