fix: create /data/compose compat symlink for legacy stacks after migration

Legacy compose stacks reference /data/compose/* for bind mounts, but
the migration only copies data to ${data_root}/compose without creating
a path alias.  When containers restart, Docker cannot resolve the
original /data/compose/... paths and fails.

After the transform successfully copies data:
1. Rename /data/compose → /data/compose.legacy-bak (preserve for rollback)
2. Create symlink /data/compose → ${data_root}/compose

This matches the /data → ${data_root} compat symlink inside the product
container.

Author: zhaojing1987

install/lib/upgrade-legacy.sh

@@ -484,6 +484,16 @@ _legacy_transform_volumes() {
     die "$EXIT_RUNTIME" "Legacy data transform failed"
   fi
   rm -rf "$tmpdir"
+
+  # After transform, create a compat symlink so legacy compose stacks
+  # that still reference /data/compose/* can find their bind-mount sources.
+  # The real data now lives under ${data_root}/compose.
+  if [ -d /data/compose ] && [ ! -L /data/compose ] && [ -d "${data_root}/compose" ]; then
+    log_step "Creating /data/compose → ${data_root}/compose compat symlink for legacy stacks"
+    mv /data/compose /data/compose.legacy-bak
+    ln -s "${data_root}/compose" /data/compose
+    log_info "Legacy /data/compose preserved at /data/compose.legacy-bak"
+  fi
 }
 
  # Import staged legacy runtime settings into the modern config model.