chore: wire ACOUSTID_API_KEY + bump backend (enrichment §1D)

- .env.example: document ACOUSTID_API_KEY (blank = embedded-tag enrichment
  only; set to enable AcoustID fingerprint identification)
- bump mcma-backend to the tag-first metadata enrichment pipeline

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

.env.example

@@ -40,7 +40,11 @@ MAX_PARALLEL_DOWNLOADS=2
 
 # external services (all optional — backend degrades gracefully if unset)
 # ML_SERVICE_URL=http://ml:9000
-# ACOUSTID_API_KEY=
+# Metadata enrichment (§1D): without a key, only embedded-tag enrichment runs;
+# set it to enable AcoustID fingerprint → MusicBrainz identification.
+# Get a free key at https://acoustid.org/new-application
+ACOUSTID_API_KEY=
+# Sent to MusicBrainz/AcoustID — put a real contact so you don't get rate-limited.
 MUSICBRAINZ_USER_AGENT=mcma-backend/0.1.0 ( https://github.com/your/repo )
 # YOUTUBE_COOKIES_PATH=/data/cookies.txt