feat(wizard): prompt for MusicBrainz/AcoustID contact email

Replace the hardcoded MUSICBRAINZ_USER_AGENT placeholder in env.template
with an optional MUSICBRAINZ_OWNER_EMAIL prompt in step_enrichment.
The backend now composes a valid User-Agent from app name + version +
this email (falling back to the project URL if left blank).

lib/env_gen.sh

@@ -84,5 +84,14 @@ generate_env() {
         } >>"$ENV_FILE"
     fi
 
+    # -- MusicBrainz/AcoustID contact (required by their usage policy) ----
+    if [[ -n "${CFG_MUSICBRAINZ_OWNER_EMAIL:-}" ]]; then
+        {
+            echo ""
+            echo "# -- MusicBrainz/AcoustID contact ----------------------------------------"
+            echo "MUSICBRAINZ_OWNER_EMAIL=${CFG_MUSICBRAINZ_OWNER_EMAIL}"
+        } >>"$ENV_FILE"
+    fi
+
     chmod 600 "$ENV_FILE"
 }