chore: serialize Terraform workflows to prevent races

Add global workflow concurrency group with queueing enabled so plan/apply/destroy runs do not overlap and contend for shared remote state.

.gitea/workflows/terraform-plan.yml

@@ -6,6 +6,10 @@ on:
       - stage
       - test
 
+concurrency:
+  group: terraform-global
+  cancel-in-progress: false
+
 jobs:
   terraform:
     name: "Terraform Plan"