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/differenceInCalendarYears.js

@@ -0,0 +1,39 @@
+import { normalizeDates } from "./_lib/normalizeDates.js";
+
+/**
+ * The {@link differenceInCalendarYears} function options.
+ */
+
+/**
+ * @name differenceInCalendarYears
+ * @category Year Helpers
+ * @summary Get the number of calendar years between the given dates.
+ *
+ * @description
+ * Get the number of calendar years between the given dates.
+ *
+ * @param laterDate - The later date
+ * @param earlierDate - The earlier date
+ * @param options - An object with options
+
+ * @returns The number of calendar years
+ *
+ * @example
+ * // How many calendar years are between 31 December 2013 and 11 February 2015?
+ * const result = differenceInCalendarYears(
+ *   new Date(2015, 1, 11),
+ *   new Date(2013, 11, 31)
+ * );
+ * //=> 2
+ */
+export function differenceInCalendarYears(laterDate, earlierDate, options) {
+  const [laterDate_, earlierDate_] = normalizeDates(
+    options?.in,
+    laterDate,
+    earlierDate,
+  );
+  return laterDate_.getFullYear() - earlierDate_.getFullYear();
+}
+
+// Fallback for modularized imports:
+export default differenceInCalendarYears;