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/isExists.d.cts

@@ -0,0 +1,29 @@
+/**
+ * @name isExists
+ * @category Common Helpers
+ * @summary Is the given date exists?
+ *
+ * @description
+ * Checks if the given arguments convert to an existing date.
+ *
+ * @param year - The year of the date to check
+ * @param month - The month of the date to check
+ * @param day - The day of the date to check
+ *
+ * @returns `true` if the date exists
+ *
+ * @example
+ * // For the valid date:
+ * const result = isExists(2018, 0, 31)
+ * //=> true
+ *
+ * @example
+ * // For the invalid date:
+ * const result = isExists(2018, 1, 31)
+ * //=> false
+ */
+export declare function isExists(
+  year: number,
+  month: number,
+  day: number,
+): boolean;