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>

backend/Dockerfile

@@ -0,0 +1,32 @@
+# NovaSonic download backend — bakes the heavy deps (ffmpeg, Deno, yt-dlp, node libs) so restarts are
+# instant instead of re-installing everything on every boot. Build context = ./backend.
+FROM node:22-slim
+
+# System deps: python (yt-dlp), ffmpeg (tagging/convert), curl/git/unzip (installers), ca-certs.
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        python3 python3-pip ffmpeg curl git unzip ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+# yt-dlp (nightly) + the Spotify helper tooling.
+RUN pip install --break-system-packages -U --pre 'yt-dlp[default]' \
+    && pip install --break-system-packages spotapi 'git+https://github.com/TzurSoffer/spotify-downloader'
+
+# Deno — yt-dlp uses it to solve YouTube's JS signature/nsig challenge.
+RUN curl -fsSL https://deno.land/install.sh | sh
+ENV PATH="/root/.deno/bin:${PATH}"
+
+# Node libs the server loads (express, kuromoji) — kept out of the app dir like the compose setup did.
+WORKDIR /deps
+RUN npm install express kuromoji
+ENV NODE_PATH=/deps/node_modules
+
+# App code.
+WORKDIR /app
+COPY . /app/
+
+ENV PORT=3700 DATA_PATH=/data
+EXPOSE 3700
+
+# Refresh yt-dlp on start (heavy deps already baked, so this is a quick pull) — non-fatal if offline,
+# the baked version still works. Keeps the "always-latest yt-dlp" behaviour of the old compose command.
+CMD ["sh","-c","pip install --break-system-packages -U --pre 'yt-dlp[default]' >/dev/null 2>&1 || true; node novasonic-server.js"]