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/monthsToYears.d.cts

@@ -0,0 +1,22 @@
+/**
+ * @name monthsToYears
+ * @category Conversion Helpers
+ * @summary Convert number of months to years.
+ *
+ * @description
+ * Convert a number of months to a full number of years.
+ *
+ * @param months - The number of months to be converted
+ *
+ * @returns The number of months converted in years
+ *
+ * @example
+ * // Convert 36 months to years:
+ * const result = monthsToYears(36)
+ * //=> 3
+ *
+ * // It uses floor rounding:
+ * const result = monthsToYears(40)
+ * //=> 3
+ */
+export declare function monthsToYears(months: number): number;