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/roundToNearestHoursWithOptions.js

@@ -0,0 +1,9 @@
+// This file is generated automatically by `scripts/build/fp.ts`. Please, don't change it.
+
+import { roundToNearestHours as fn } from "../roundToNearestHours.js";
+import { convertToFP } from "./_lib/convertToFP.js";
+
+export const roundToNearestHoursWithOptions = convertToFP(fn, 2);
+
+// Fallback for modularized imports:
+export default roundToNearestHoursWithOptions;