fix(install): back up current binary and GC stale artefacts on upgrade

Before overwriting the binary, read its version via --version and save
a copy as aftertouch-service.<version>.backup (falls back to a timestamp
if the flag is absent or the build is a dev build).

After the new binary is in place, delete every older *.backup, *.old,
and *.new artefact in INSTALL_DIR.  /mnt/nv on SoundTouch SCM modules
has only tens of MB free; accumulating one ~12 MB backup per upgrade
quickly causes 'no space left on device' on the next download.

Only the backup created in this run (the <current-release>-1 binary) is
kept, giving a single one-step rollback point without wasting disk.

Relates to #329 (on-device install friction reported by weissigera).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/on-device-install/install.sh

@@ -44,9 +44,40 @@ curl \
   --fail \
   "$BINARY_URL"
 
+# Back up the current binary before overwriting so a one-step rollback
+# is always available.  The version string comes from the binary itself;
+# if it is absent (very old build or corrupted) we fall back to a timestamp.
+BACKUP_FILE=""
+if [ -f "$INSTALL_DIR/aftertouch-service" ]; then
+  current_version=$("$INSTALL_DIR/aftertouch-service" --version 2>/dev/null \
+    | awk '{print $NF}') || true
+  if [ -z "$current_version" ] || [ "$current_version" = "dev" ]; then
+    current_version=$(date +%Y%m%d-%H%M%S)
+  fi
+  BACKUP_FILE="$INSTALL_DIR/aftertouch-service.${current_version}.backup"
+  cp -p "$INSTALL_DIR/aftertouch-service" "$BACKUP_FILE"
+  echo "Backed up current binary ($current_version) → $BACKUP_FILE"
+fi
+
 mv "$UPDATE_TMP_DIR/binary" "$INSTALL_DIR/aftertouch-service"
 chmod +x "$INSTALL_DIR/aftertouch-service"
 
+# Keep only the backup we just created; prune all older *.backup, *.old, and
+# *.new artefacts left by earlier installs.  /mnt/nv is small (tens of MB),
+# so accumulation quickly causes "no space left on device" during downloads.
+if [ -n "$BACKUP_FILE" ]; then
+  echo "Disk usage before GC:"; df -h "$INSTALL_DIR"
+  for f in "$INSTALL_DIR/aftertouch-service".*.backup \
+            "$INSTALL_DIR/aftertouch-service".*.old \
+            "$INSTALL_DIR/aftertouch-service.new"; do
+    [ -f "$f" ] || continue
+    [ "$f" = "$BACKUP_FILE" ] && continue
+    rm -f "$f"
+    echo "Removed stale artefact: $f"
+  done
+  echo "Disk usage after GC:"; df -h "$INSTALL_DIR"
+fi
+
 echo "Creating init script..."
 curl \
   -sSL \