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/getMilliseconds.js

@@ -0,0 +1,25 @@
+import { toDate } from "./toDate.js";
+
+/**
+ * @name getMilliseconds
+ * @category Millisecond Helpers
+ * @summary Get the milliseconds of the given date.
+ *
+ * @description
+ * Get the milliseconds of the given date.
+ *
+ * @param date - The given date
+ *
+ * @returns The milliseconds
+ *
+ * @example
+ * // Get the milliseconds of 29 February 2012 11:45:05.123:
+ * const result = getMilliseconds(new Date(2012, 1, 29, 11, 45, 5, 123))
+ * //=> 123
+ */
+export function getMilliseconds(date) {
+  return toDate(date).getMilliseconds();
+}
+
+// Fallback for modularized imports:
+export default getMilliseconds;