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/isDate.js

@@ -0,0 +1,42 @@
+/**
+ * @name isDate
+ * @category Common Helpers
+ * @summary Is the given value a date?
+ *
+ * @description
+ * Returns true if the given value is an instance of Date. The function works for dates transferred across iframes.
+ *
+ * @param value - The value to check
+ *
+ * @returns True if the given value is a date
+ *
+ * @example
+ * // For a valid date:
+ * const result = isDate(new Date())
+ * //=> true
+ *
+ * @example
+ * // For an invalid date:
+ * const result = isDate(new Date(NaN))
+ * //=> true
+ *
+ * @example
+ * // For some value:
+ * const result = isDate('2014-02-31')
+ * //=> false
+ *
+ * @example
+ * // For an object:
+ * const result = isDate({})
+ * //=> false
+ */
+export function isDate(value) {
+  return (
+    value instanceof Date ||
+    (typeof value === "object" &&
+      Object.prototype.toString.call(value) === "[object Date]")
+  );
+}
+
+// Fallback for modularized imports:
+export default isDate;