fix(ci): add concurrency group to cancel stale runs on same branch

Prevents double-run on PR (push + pull_request events), eliminates
cache contention, and frees runner capacity from queued feature-branch jobs.

.github/workflows/ci.yml

@@ -15,6 +15,10 @@ on:
   pull_request:
     branches: [main]
 
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   backend:
     runs-on: ubuntu-latest