Use static concurrency group for Fly.io deployments

This ensures all deployments to the same Fly.io app are serialized,
regardless of branch or trigger source.

Co-authored-by: spawnia <12158000+spawnia@users.noreply.github.com>

Author: Copilot

.github/workflows/deploy.yml

@@ -6,7 +6,7 @@ on:
       - master
 
 concurrency:
-  group: deploy-${{ github.ref }}
+  group: deploy-fly-io-production
   cancel-in-progress: false
 
 env: