refactor(docker): make image-based the default compose, build variant is .dev

Less confusion: docker-compose.yml is now the default (pulls pre-built images)
and the build-on-host variant is docker-compose.dev.yml. Updated all references
(CLAUDE.md, README, .env.example, build-push-images.mjs, compose header).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -41,13 +41,14 @@ npm run build    # production build → dist/
 npm run preview  # preview the production build
 ```
 
-Backend runs in Docker. Two ways:
-- **`docker-compose.yml`** — builds on the host (mounts `./backend`, installs deps at container
-  start). Restart after backend changes: `docker compose restart novasonic-backend` (re-pulls yt-dlp).
-- **`docker-compose.prod.yml`** — PULLS pre-built images (deps baked into `backend/Dockerfile` +
-  `scrobble/Dockerfile`), so restarts are instant. Build+push from dev with `npm run push-images`
-  (needs `REGISTRY`/`TAG` in `.env`), then on the host `docker compose -f docker-compose.prod.yml pull
-  && … up -d`. See `.env.example`.
+Backend runs in Docker. Two compose files:
+- **`docker-compose.yml`** (default/production) — PULLS pre-built images (deps baked into
+  `backend/Dockerfile` + `scrobble/Dockerfile`), so restarts are instant. Build+push from dev with
+  `npm run push-images` (needs `REGISTRY`/`TAG` in `.env`), then on the host `docker compose pull &&
+  docker compose up -d`. See `.env.example`.
+- **`docker-compose.dev.yml`** — builds on the host (mounts `./backend`, installs deps at container
+  start). Restart after backend changes: `docker compose -f docker-compose.dev.yml restart
+  novasonic-backend` (re-pulls yt-dlp).
 
 On the deploy host the music lives at `/opt/navidrome/music`.