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/caniuse-lite/dist/unpacker/features.js

@@ -0,0 +1,6 @@
+/*
+ * Load this dynamically so that it
+ * doesn't appear in the rollup bundle.
+ */
+
+module.exports.features = require('../../data/features')