diff --git a/.env.example b/.env.example
index cf368fd..8e90669 100644
--- a/.env.example
+++ b/.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
 
diff --git a/mcma-backend b/mcma-backend
index 4ade693..c72d195 160000
--- a/mcma-backend
+++ b/mcma-backend
@@ -1 +1 @@
-Subproject commit 4ade6939b60fe46350c50c4b56b566d28113aa52
+Subproject commit c72d19599a85ca7abe636e2e30eb33b1325722fc