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/differenceInMonths.d.cts

@@ -0,0 +1,26 @@
+import type { ContextOptions, DateArg } from "./types.js";
+/**
+ * The {@link differenceInMonths} function options.
+ */
+export interface DifferenceInMonthsOptions extends ContextOptions<Date> {}
+/**
+ * @name differenceInMonths
+ * @category Month Helpers
+ * @summary Get the number of full months between the given dates.
+ *
+ * @param laterDate - The later date
+ * @param earlierDate - The earlier date
+ * @param options - An object with options
+ *
+ * @returns The number of full months
+ *
+ * @example
+ * // How many full months are between 31 January 2014 and 1 September 2014?
+ * const result = differenceInMonths(new Date(2014, 8, 1), new Date(2014, 0, 31))
+ * //=> 7
+ */
+export declare function differenceInMonths(
+  laterDate: DateArg<Date> & {},
+  earlierDate: DateArg<Date> & {},
+  options?: DifferenceInMonthsOptions | undefined,
+): number;