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/fp/nextDay.d.ts

@@ -0,0 +1,5 @@
+export declare const nextDay: import("./types.js").FPFn2<
+  Date,
+  import("../fp.js").Day,
+  import("../fp.js").DateArg<Date>
+>;