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/differenceInHours.d.ts

@@ -0,0 +1,34 @@
+import type { ContextOptions, DateArg, RoundingOptions } from "./types.js";
+/**
+ * The {@link differenceInHours} function options.
+ */
+export interface DifferenceInHoursOptions
+  extends RoundingOptions,
+    ContextOptions<Date> {}
+/**
+ * @name differenceInHours
+ * @category Hour Helpers
+ * @summary Get the number of hours between the given dates.
+ *
+ * @description
+ * Get the number of hours between the given dates.
+ *
+ * @param laterDate - The later date
+ * @param earlierDate - The earlier date
+ * @param options - An object with options.
+ *
+ * @returns The number of hours
+ *
+ * @example
+ * // How many hours are between 2 July 2014 06:50:00 and 2 July 2014 19:00:00?
+ * const result = differenceInHours(
+ *   new Date(2014, 6, 2, 19, 0),
+ *   new Date(2014, 6, 2, 6, 50)
+ * )
+ * //=> 12
+ */
+export declare function differenceInHours(
+  laterDate: DateArg<Date> & {},
+  earlierDate: DateArg<Date> & {},
+  options?: DifferenceInHoursOptions,
+): number;