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/tailwindcss/dist/plugin.js

@@ -0,0 +1 @@
+"use strict";function g(i,n){return{handler:i,config:n}}g.withOptions=function(i,n=()=>({})){function t(o){return{handler:i(o),config:n(o)}}return t.__isOptionsFunction=!0,t};var u=g;module.exports=u;