refactor(docker): default compose builds from Dockerfiles (baked deps, cached)

Chosen flow: build on the host from backend/scrobble Dockerfiles instead of
pulling from a registry (no registry / no Docker-on-dev needed). Heavy deps are
baked, so restarts are instant and rebuilds after git pull are cache-fast.
Removed the now-obsolete docker-compose.dev.yml (runtime-install variant); the
optional push-to-registry script stays for a future CI flow. Updated README,
CLAUDE.md, .env.example.

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

.env.example

@@ -1,5 +1,6 @@
 # Copy to .env next to your docker-compose file and fill in.  cp .env.example .env
-# Used by docker-compose.yml (pulls pre-built images) AND docker-compose.dev.yml (build-on-host).
+# Used by docker-compose.yml (builds locally from the Dockerfiles).
+# The REGISTRY/TAG at the bottom are only for the optional pull-from-registry / CI flow.
 
 # ── URLs pre-filled on the frontend setup screen ─────────────────────────────
 NAVIDROME_URL=https://music.example.com