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/secondsToMilliseconds.d.ts

@@ -0,0 +1,18 @@
+/**
+ * @name secondsToMilliseconds
+ * @category Conversion Helpers
+ * @summary Convert seconds to milliseconds.
+ *
+ * @description
+ * Convert a number of seconds to a full number of milliseconds.
+ *
+ * @param seconds - The number of seconds to be converted
+ *
+ * @returns The number of seconds converted in milliseconds
+ *
+ * @example
+ * // Convert 2 seconds into milliseconds
+ * const result = secondsToMilliseconds(2)
+ * //=> 2000
+ */
+export declare function secondsToMilliseconds(seconds: number): number;