fix: storage plugin install setup and post-delete permissions

apps:destroy was failing in CI because the post-delete trigger called PropertyListWrite, which tried to mkdir /var/lib/dokku/config/storage/<app> as the dokku user but the parent directory had been created by the install trigger as root. The install trigger now calls PropertySetup("storage") so the per-plugin config root has the correct ownership, and post-delete / post-app-clone / post-app-rename short-circuit when the app has no attachments to begin with so they can't fail on a never-touched property tree. The entry registry and migration flag directories also moved out of config/storage (where they would have collided with the property-store path config/storage/<appName>) into a dedicated data/storage-registry tree.

plugins/storage/entry_test.go

@@ -138,7 +138,7 @@ func TestEntryRoundTrip(t *testing.T) {
 	Expect(loaded.Labels).To(Equal(original.Labels))
 	Expect(loaded.SchemaVersion).To(Equal(SchemaVersion))
 
-	expectedPath := filepath.Join(root, "config", "storage", "entries", "demo-data.json")
+	expectedPath := filepath.Join(root, "data", "storage-registry", "entries", "demo-data.json")
 	Expect(expectedPath).To(BeARegularFile())
 
 	Expect(DeleteEntry("demo-data")).To(Succeed())