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/addLeadingZeros.cjs

@@ -0,0 +1,7 @@
+"use strict";
+exports.addLeadingZeros = addLeadingZeros;
+function addLeadingZeros(number, targetLength) {
+  const sign = number < 0 ? "-" : "";
+  const output = Math.abs(number).toString().padStart(targetLength, "0");
+  return sign + output;
+}