fix(ci): separate concurrency groups so polling is not blocked by builds

poll-upstream and build-release shared the poll-and-build concurrency
group, which caused cron-triggered poll runs to be queued or dropped
while a build was in progress. Give each workflow its own group and
set cancel-in-progress: true for the poller since stale polls are
redundant.

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

.gitea/workflows/build-release.yml

@@ -9,7 +9,7 @@ on:
         type: string
 
 concurrency:
-  group: poll-and-build
+  group: build-release
   cancel-in-progress: false
 
 jobs:

.gitea/workflows/poll-upstream.yml

@@ -6,8 +6,8 @@ on:
   workflow_dispatch: {}
 
 concurrency:
-  group: poll-and-build
-  cancel-in-progress: false
+  group: poll-upstream
+  cancel-in-progress: true
 
 jobs:
   check: