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/differenceInMilliseconds.cjs

@@ -0,0 +1,29 @@
+"use strict";
+exports.differenceInMilliseconds = differenceInMilliseconds;
+var _index = require("./toDate.cjs");
+
+/**
+ * @name differenceInMilliseconds
+ * @category Millisecond Helpers
+ * @summary Get the number of milliseconds between the given dates.
+ *
+ * @description
+ * Get the number of milliseconds between the given dates.
+ *
+ * @param laterDate - The later date
+ * @param earlierDate - The earlier date
+ *
+ * @returns The number of milliseconds
+ *
+ * @example
+ * // How many milliseconds are between
+ * // 2 July 2014 12:30:20.600 and 2 July 2014 12:30:21.700?
+ * const result = differenceInMilliseconds(
+ *   new Date(2014, 6, 2, 12, 30, 21, 700),
+ *   new Date(2014, 6, 2, 12, 30, 20, 600)
+ * )
+ * //=> 1100
+ */
+function differenceInMilliseconds(laterDate, earlierDate) {
+  return +(0, _index.toDate)(laterDate) - +(0, _index.toDate)(earlierDate);
+}