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.js

@@ -0,0 +1,29 @@
+import { toDate } from "../toDate.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 function getTimezoneOffsetInMilliseconds(date) {
+  const _date = toDate(date);
+  const utcDate = new Date(
+    Date.UTC(
+      _date.getFullYear(),
+      _date.getMonth(),
+      _date.getDate(),
+      _date.getHours(),
+      _date.getMinutes(),
+      _date.getSeconds(),
+      _date.getMilliseconds(),
+    ),
+  );
+  utcDate.setUTCFullYear(_date.getFullYear());
+  return +date - +utcDate;
+}