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/millisecondsToSeconds.d.cts

@@ -0,0 +1,23 @@
+/**
+ * @name millisecondsToSeconds
+ * @category Conversion Helpers
+ * @summary Convert milliseconds to seconds.
+ *
+ * @description
+ * Convert a number of milliseconds to a full number of seconds.
+ *
+ * @param milliseconds - The number of milliseconds to be converted
+ *
+ * @returns The number of milliseconds converted in seconds
+ *
+ * @example
+ * // Convert 1000 milliseconds to seconds:
+ * const result = millisecondsToSeconds(1000)
+ * //=> 1
+ *
+ * @example
+ * // It uses floor rounding:
+ * const result = millisecondsToSeconds(1999)
+ * //=> 1
+ */
+export declare function millisecondsToSeconds(milliseconds: number): number;