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.cjs

@@ -0,0 +1,12 @@
+"use strict";
+exports.normalizeInterval = normalizeInterval;
+var _index = require("./normalizeDates.cjs");
+
+function normalizeInterval(context, interval) {
+  const [start, end] = (0, _index.normalizeDates)(
+    context,
+    interval.start,
+    interval.end,
+  );
+  return { start, end };
+}