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/react-router-dom/README.md

@@ -0,0 +1,6 @@
+This package simply re-exports everything from `react-router` to smooth the upgrade path for v6 applications. Once upgraded you can change all of your imports and remove it from your dependencies:
+
+```diff
+-import { Routes } from "react-router-dom"
++import { Routes } from "react-router"
+```