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/hoursToSeconds.js

@@ -0,0 +1,25 @@
+import { secondsInHour } from "./constants.js";
+
+/**
+ * @name hoursToSeconds
+ * @category Conversion Helpers
+ * @summary Convert hours to seconds.
+ *
+ * @description
+ * Convert a number of hours to a full number of seconds.
+ *
+ * @param hours - The number of hours to be converted
+ *
+ * @returns The number of hours converted in seconds
+ *
+ * @example
+ * // Convert 2 hours to seconds:
+ * const result = hoursToSeconds(2)
+ * //=> 7200
+ */
+export function hoursToSeconds(hours) {
+  return Math.trunc(hours * secondsInHour);
+}
+
+// Fallback for modularized imports:
+export default hoursToSeconds;