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/scheduler/README.md

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+# `scheduler`
+
+This is a package for cooperative scheduling in a browser environment. It is currently used internally by React, but we plan to make it more generic.
+
+The public API for this package is not yet finalized.
+
+### Thanks
+
+The React team thanks [Anton Podviaznikov](https://podviaznikov.com/) for donating the `scheduler` package name.