fix(ci): add concurrency to prevent deploy race conditions

Two simultaneous pushes (local + Forgejo mirror) can trigger parallel
workflow runs. The older commit's run finishing last overwrites the
correct deploy. cancel-in-progress ensures only the latest wins.

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

Author: ferr079

.github/workflows/deploy.yml

@@ -4,6 +4,10 @@ on:
   push:
     branches: [main]
 
+concurrency:
+  group: deploy
+  cancel-in-progress: true
+
 jobs:
   deploy:
     runs-on: ubuntu-latest