fix(queue): use label= (singular) not labels= (plural) for Gitea 1.22.6 API

Gitea 1.22.6's /repos/{R}/issues endpoint accepts `label` (singular)
for filtering by a single label name, not `labels` (plural). Using the
wrong param silently returns 0 results, so the queue cron has been
picking no PRs for every tick.

Fixes the merge queue silently returning 0 queued PRs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/scripts/gitea-merge-queue.py

@@ -284,7 +284,8 @@ def list_queued_issues() -> list[dict]:
         query={
             "state": "open",
             "type": "pulls",
-            "labels": QUEUE_LABEL,
+            # NOTE: Gitea 1.22.6 uses `label` (singular), not `labels` (plural).
+            "label": QUEUE_LABEL,
             "limit": "50",
         },
     )