feat(docker): pull-based prod stack (docker-compose.prod.yml + Registry-Images)

Produktivsystem baut nicht mehr selbst, sondern zieht die drei fertigen Images
(novasonic-frontend/-backend/-scrobble) aus der Gitea-Registry. Das bestehende
docker-compose.yml bleibt der Build-auf-dem-Host-Weg. Dazu .dockerignore fuers
Frontend-Image (android/, backend/, .git raus) und README/CLAUDE.md aktualisiert.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -88,6 +88,7 @@ NovaSonic/
 ├── vite.config.js
 ├── Dockerfile
 ├── docker-compose.yml
+├── docker-compose.prod.yml     # Produktiv: zieht fertige Images aus der Registry
 ├── nginx.conf
 └── docker-entrypoint.sh       # Injects NAVIDROME_URL / BACKEND_URL / SCROBBLE_URL at runtime
 ```
@@ -127,8 +128,19 @@ docker compose up -d --build
 ```
 
 Builds locally from the Dockerfiles (deps baked in → instant restarts; rebuilds after a `git pull`
-are fast thanks to Docker's layer cache). No registry needed. (An optional pull-from-registry / CI
-flow exists too — `npm run push-images` + `REGISTRY`/`TAG` in `.env`, see `.env.example`.)
+are fast thanks to Docker's layer cache). No registry needed.
+
+**Produktiv ohne Bauen** — die drei Images kommen fertig aus der Registry:
+
+```bash
+docker compose -f docker-compose.prod.yml pull
+docker compose -f docker-compose.prod.yml up -d
+```
+
+Dafür in der `.env` `REGISTRY` (und optional `TAG`) setzen; die Images heißen
+`$REGISTRY/novasonic-frontend|-backend|-scrobble`. Neue Stände von der Dev-Maschine aus mit
+`npm run push-images` (bzw. `docker build -t $REGISTRY/<name>:latest <context> && docker push …`)
+hochladen — Kontexte: `.` (frontend), `backend/`, `scrobble/`.
 
 Frontend on port 8080, backend on port 3700, scrobble service on port 3710. Route each through your reverse proxy. **Important:** the scrobble subdomain must *not* sit behind an SSO / forward-auth gateway — it authenticates itself via Navidrome (`ping.view`), and a gateway in front redirects the app's API calls to a login page.