CI: one e2e at a time across every PR

Gitea now has two runners but there is one 4090. The e2e job joins one
concurrency group without cancel-in-progress, so a second PR's e2e waits
for the first. checks still run in parallel.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

.gitea/workflows/ci.yml

@@ -108,6 +108,13 @@ jobs:
       || contains(github.event.pull_request.labels.*.name, 'e2e:full'))
     needs: checks
     runs-on: ubuntu-latest
+    # One e2e at a time across every PR and branch: there are two runners
+    # but one 4090. A second waits for the first instead of sharing the GPU.
+    # A group holds one running and one waiting job, so a third PR's e2e
+    # replaces the waiting one, which shows as cancelled: rerun it.
+    concurrency:
+      group: nisse-e2e
+      cancel-in-progress: false
     timeout-minutes: 90
     steps:
       - uses: actions/checkout@v4