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/@babel/compat-data/corejs3-shipped-proposals.js

@@ -0,0 +1,2 @@
+// Todo (Babel 8): remove this file now that it is included in babel-plugin-polyfill-corejs3
+module.exports = require("./data/corejs3-shipped-proposals.json");