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"
 }

lib/ui.sh

@@ -132,3 +132,4 @@ v_port()     { [[ "$1" =~ ^[0-9]+$ ]] && (($1 >= 1 && $1 <= 65535)) || { ui_warn
 v_url()      { [[ "$1" =~ ^https?:// ]] || { ui_warn "$(t invalid_input)"; return 1; }; }
 v_redis()    { [[ "$1" =~ ^rediss?:// ]] || { ui_warn "$(t invalid_input)"; return 1; }; }
 v_domain()   { [[ "$1" =~ ^[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ ]] || { ui_warn "$(t invalid_input)"; return 1; }; }
+v_email_opt() { [[ -z "$1" || "$1" =~ ^[^[:space:]@]+@[^[:space:]@]+\.[a-zA-Z]{2,}$ ]] || { ui_warn "$(t invalid_input)"; return 1; }; }