ci: single Shopware install into shopware_test; serialize releases

DATABASE_URL already ends in _test so the TestBootstrapper finds an installed DB and
skips its own migration run; FORCE_INSTALL (which failed with an unknown database) removed.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -12,6 +12,11 @@ permissions:
   contents: write
   packages: write
 
+# Never run two releases at once; a second push waits instead of racing on the tag.
+concurrency:
+  group: release
+  cancel-in-progress: false
+
 jobs:
   release:
     runs-on: ubuntu-latest