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/isValid.js

@@ -0,0 +1,40 @@
+import { isDate } from "./isDate.js";
+import { toDate } from "./toDate.js";
+
+/**
+ * @name isValid
+ * @category Common Helpers
+ * @summary Is the given date valid?
+ *
+ * @description
+ * Returns false if argument is Invalid Date and true otherwise.
+ * Argument is converted to Date using `toDate`. See [toDate](https://date-fns.org/docs/toDate)
+ * Invalid Date is a Date, whose time value is NaN.
+ *
+ * Time value of Date: http://es5.github.io/#x15.9.1.1
+ *
+ * @param date - The date to check
+ *
+ * @returns The date is valid
+ *
+ * @example
+ * // For the valid date:
+ * const result = isValid(new Date(2014, 1, 31))
+ * //=> true
+ *
+ * @example
+ * // For the value, convertible into a date:
+ * const result = isValid(1393804800000)
+ * //=> true
+ *
+ * @example
+ * // For the invalid date:
+ * const result = isValid(new Date(''))
+ * //=> false
+ */
+export function isValid(date) {
+  return !((!isDate(date) && typeof date !== "number") || isNaN(+toDate(date)));
+}
+
+// Fallback for modularized imports:
+export default isValid;