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/.env.example

@@ -0,0 +1,7 @@
+# Dev proxy target — Vite proxies /auth, /events, etc. to this host.
+# Change to match your API SERVER_PORT if you changed it.
+VITE_API_URL=http://localhost:8080
+
+# Production only — set this when the frontend and API are on different origins.
+# Leave unset if the API serves the frontend (same origin).
+# VITE_API_BASE=https://api.example.com