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

tests/ci/test_workflow.py

@@ -0,0 +1,16 @@
+"""The CI workflow's settings that matter beyond a single run."""
+
+from pathlib import Path
+
+import yaml
+
+CI = Path(__file__).parent.parent.parent / ".gitea" / "workflows" / "ci.yml"
+
+
+def test_only_one_e2e_runs_at_a_time() -> None:
+    """Two runners, one 4090: e2e runs share one group across every PR,
+    and a run waits for the one before rather than cancelling it."""
+    e2e = yaml.safe_load(CI.read_text())["jobs"]["e2e"]
+    group = e2e["concurrency"]["group"]
+    assert "${{" not in group  # the same group for every PR and branch
+    assert e2e["concurrency"]["cancel-in-progress"] is False