fix(ci): prevent poll-upstream from cancelling in-progress builds

Poll was firing every minute, dispatching new builds that cancelled
the running one. Restore 15-minute cron interval and add shared
concurrency group across both workflows so new polls queue instead
of re-dispatching.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/poll-upstream.yml

@@ -2,9 +2,13 @@ name: poll-upstream
 
 on:
   schedule:
-    - cron: "*/1 * * * *"
+    - cron: "*/15 * * * *"
   workflow_dispatch: {}
 
+concurrency:
+  group: poll-and-build
+  cancel-in-progress: false
+
 jobs:
   check:
     runs-on: fedora