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/helper-compilation-targets/README.md

@@ -0,0 +1,19 @@
+# @babel/helper-compilation-targets
+
+> Helper functions on Babel compilation targets
+
+See our website [@babel/helper-compilation-targets](https://babeljs.io/docs/babel-helper-compilation-targets) for more information.
+
+## Install
+
+Using npm:
+
+```sh
+npm install --save @babel/helper-compilation-targets
+```
+
+or using yarn:
+
+```sh
+yarn add @babel/helper-compilation-targets
+```