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/build-release.yml

@@ -8,6 +8,10 @@ on:
         required: true
         type: string
 
+concurrency:
+  group: poll-and-build
+  cancel-in-progress: false
+
 jobs:
   build:
     strategy:
@@ -106,11 +110,6 @@ jobs:
   publish:
     needs: package
     runs-on: fedora
-    # concurrency ensures only one publish runs at a time — repo metadata
-    # corruption is a nightmare if two createrepo_c processes race.
-    concurrency:
-      group: rpm-publish
-      cancel-in-progress: false
     steps:
       - uses: actions/checkout@v4