feat(docker): image-based prod stack (Dockerfiles + prod compose + push script)

Adds a pull-based production option so the host no longer builds/installs deps
on every restart:
- backend/Dockerfile + scrobble/Dockerfile bake the heavy deps (ffmpeg, Deno,
  yt-dlp, node libs). Backend still refreshes yt-dlp on start (non-fatal).
- docker-compose.prod.yml pulls ${REGISTRY}/novasonic-*:${TAG}.
- scripts/build-push-images.mjs (npm run push-images) builds + pushes all three.
- .env.example documents every var incl. REGISTRY/TAG; .env gitignored.

The build-on-host docker-compose.yml stays as-is.

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

.gitignore

@@ -3,3 +3,4 @@
 node_modules/
 dist/
 scripts/.env.release
+.env