ci: Add concurrency groups to prevent deployment conflicts (#381)

## Summary

Add per-environment concurrency groups to staging and production
deployment jobs to prevent Pulumi stack lock conflicts when multiple PRs
are merged simultaneously.

- `deploy-staging` job gets `deploy-staging` concurrency group
- `deploy-production` job gets `deploy-production` concurrency group  
- Both use `cancel-in-progress: false` to queue deployments safely

This eliminates CI noise from failed parallel deployments while
maintaining deployment reliability.

Fixes #375

.github/workflows/deploy.yml

@@ -56,6 +56,9 @@ jobs:
     runs-on: ubuntu-latest
     environment: staging
     needs: docker-push
+    concurrency:
+      group: deploy-staging
+      cancel-in-progress: false
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -92,6 +95,9 @@ jobs:
     runs-on: ubuntu-latest
     environment: production
     needs: deploy-staging
+    concurrency:
+      group: deploy-production
+      cancel-in-progress: false
     steps:
       - name: Checkout code
         uses: actions/checkout@v4