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/escalade/index.d.ts

@@ -0,0 +1,15 @@
+type Promisable<T> = T | Promise<T>;
+
+declare namespace escalade {
+	export type Callback = (
+		directory: string,
+		files: string[],
+	) => Promisable<string | false | void>;
+}
+
+declare function escalade(
+	directory: string,
+	callback: escalade.Callback,
+): Promise<string | void>;
+
+export = escalade;