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/locale/zh-CN.d.ts

@@ -0,0 +1,13 @@
+import type { Locale } from "./types.js";
+/**
+ * @category Locales
+ * @summary Chinese Simplified locale.
+ * @language Chinese Simplified
+ * @iso-639-2 zho
+ * @author Changyu Geng [@KingMario](https://github.com/KingMario)
+ * @author Song Shuoyun [@fnlctrl](https://github.com/fnlctrl)
+ * @author sabrinaM [@sabrinamiao](https://github.com/sabrinamiao)
+ * @author Carney Wu [@cubicwork](https://github.com/cubicwork)
+ * @author Terrence Lam [@skyuplam](https://github.com/skyuplam)
+ */
+export declare const zhCN: Locale;