ci: add concurrency control to image-only.yml

Two rapid commits today (2acba0a → c5ed556) queued back-to-back
runs of the same workflow. Each run is ~90 min (the reindex is the
long pole), and the older commit's image just gets overwritten by
the newer one anyway — so the older run is pure waste.

  concurrency:
    group: image-only
    cancel-in-progress: true

cancels any in-flight run on the same workflow when a new push
lands. Future-me will thank present-me.

(The workflow-file change doesn't itself trigger image-only.yml
since .gitea/workflows/** isn't in the paths: filter — so this
commit doesn't kick off a third run.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/image-only.yml

@@ -20,6 +20,13 @@ on:
       - "Dockerfile"
       - "sources.json"
 
+# If multiple pushes land in quick succession, cancel the older one
+# rather than queueing both — each run is ~90 min and the older
+# commit's image just gets overwritten by the newer one anyway.
+concurrency:
+  group: image-only
+  cancel-in-progress: true
+
 env:
   REGISTRY_PUSH: 192.168.0.2:1234
   REGISTRY_PULL: git.jpaul.io