feat: first-run web installer with admin account creation

- Backend: app_settings table + users.is_admin flag (schema + migrate)
- POST /api/setup creates admin user, first workspace and marks installed
  (transactional, single-use, rate-limited); GET /api/setup/status
- Guard middleware: API answers 503 { setupRequired: true } until installed
- Frontend: /setup wizard (welcome, admin account, workspace), router guard
  redirects to installer when uninstalled, API interceptor handles 503
- Docs: installer covered in README, SETUP, HOSTING and ROADMAP

docs/HOSTING.md

@@ -122,6 +122,9 @@ curl http://localhost:3000/api/health
 - Frontend: `http://<host>:8080`
 - Backend API: `http://<host>:3000/api/health`
 - Data persists in the `pgdata` volume.
+- On first start, open the frontend and complete the **web installer**
+  (admin account + first workspace). Until then the API answers
+  `503 { setupRequired: true }` for all app routes.
 
 ### Reverse proxy (TLS) in front of Compose
 

docs/ROADMAP.md

@@ -3,6 +3,7 @@
 ## Phase 1 — Web app MVP (this repo, v0.1) ✅
 
 - [x] Auth (register/login/JWT), workspaces, invites, roles
+- [x] First-run web installer (admin account + first workspace, API locked until done)
 - [x] Nested pages, TipTap rich text, comments, search
 - [x] Socket.IO realtime (live updates, presence, typing)
 - [x] Light/dark mode

docs/SETUP.md

@@ -57,8 +57,10 @@ npm install
 npm run dev   # http://localhost:5173
 ```
 
-Open the frontend, **Register** the first user, create a workspace, invite teammates
-by email (they must register with the same email first).
+Open the frontend, where the **first-run web installer** (`/setup`) creates the admin
+account (`is_admin=true`) and the first workspace, then logs you in automatically.
+Further teammates **Register** themselves and are invited by email
+(they must register with the same email first).
 
 ## Troubleshooting