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/getTimezoneOffsetInMilliseconds.d.ts

@@ -0,0 +1,15 @@
+import type { DateArg } from "../types.js";
+/**
+ * Google Chrome as of 67.0.3396.87 introduced timezones with offset that includes seconds.
+ * They usually appear for dates that denote time before the timezones were introduced
+ * (e.g. for 'Europe/Prague' timezone the offset is GMT+00:57:44 before 1 October 1891
+ * and GMT+01:00:00 after that date)
+ *
+ * Date#getTimezoneOffset returns the offset in minutes and would return 57 for the example above,
+ * which would lead to incorrect calculations.
+ *
+ * This function returns the timezone offset in milliseconds that takes seconds in account.
+ */
+export declare function getTimezoneOffsetInMilliseconds(
+  date: DateArg<Date> & {},
+): number;