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/yearsToMonths.d.ts

@@ -0,0 +1,18 @@
+/**
+ * @name yearsToMonths
+ * @category Conversion Helpers
+ * @summary Convert years to months.
+ *
+ * @description
+ * Convert a number of years to a full number of months.
+ *
+ * @param years - The number of years to be converted
+ *
+ * @returns The number of years converted in months
+ *
+ * @example
+ * // Convert 2 years into months
+ * const result = yearsToMonths(2)
+ * //=> 24
+ */
+export declare function yearsToMonths(years: number): number;