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/_lib/defaultLocale.cjs

@@ -0,0 +1,8 @@
+"use strict";
+Object.defineProperty(exports, "defaultLocale", {
+  enumerable: true,
+  get: function () {
+    return _index.enUS;
+  },
+});
+var _index = require("../locale/en-US.cjs");