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/differenceInSeconds.d.cts

@@ -0,0 +1,33 @@
+import type { DateArg, RoundingOptions } from "./types.js";
+/**
+ * The {@link differenceInSeconds} function options.
+ */
+export interface DifferenceInSecondsOptions extends RoundingOptions {}
+/**
+ * @name differenceInSeconds
+ * @category Second Helpers
+ * @summary Get the number of seconds between the given dates.
+ *
+ * @description
+ * Get the number of seconds between the given dates.
+ *
+ * @param laterDate - The later date
+ * @param earlierDate - The earlier date
+ * @param options - An object with options.
+ *
+ * @returns The number of seconds
+ *
+ * @example
+ * // How many seconds are between
+ * // 2 July 2014 12:30:07.999 and 2 July 2014 12:30:20.000?
+ * const result = differenceInSeconds(
+ *   new Date(2014, 6, 2, 12, 30, 20, 0),
+ *   new Date(2014, 6, 2, 12, 30, 7, 999)
+ * )
+ * //=> 12
+ */
+export declare function differenceInSeconds(
+  laterDate: DateArg<Date> & {},
+  earlierDate: DateArg<Date> & {},
+  options?: DifferenceInSecondsOptions,
+): number;