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>

scrobble/Dockerfile

@@ -0,0 +1,15 @@
+# NovaSonic scrobble service — isolated timestamped play-log. Uses Node's built-in SQLite
+# (--experimental-sqlite), so the only dependency is express. Build context = ./scrobble.
+FROM node:22-slim
+
+WORKDIR /deps
+RUN npm install express
+ENV NODE_PATH=/deps/node_modules
+
+WORKDIR /app
+COPY . /app/
+
+ENV PORT=3710 DATA_PATH=/data
+EXPOSE 3710
+
+CMD ["node","--experimental-sqlite","scrobble-server.js"]