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/normalizeInterval.js

@@ -0,0 +1,6 @@
+import { normalizeDates } from "./normalizeDates.js";
+
+export function normalizeInterval(context, interval) {
+  const [start, end] = normalizeDates(context, interval.start, interval.end);
+  return { start, end };
+}