ci: add explicit permissions to workflows

Fixes CodeQL 'Workflow does not contain permissions' warnings.
Sets minimal 'contents: read' at top level.

.github/workflows/docker-build.yml

@@ -11,6 +11,10 @@ on:
         required: false
         default: ''
 
+# Default minimal permissions
+permissions:
+  contents: read
+
 env:
   REGISTRY: ghcr.io
 

.github/workflows/test.yml

@@ -4,6 +4,10 @@ on:
   pull_request:
     branches: [main]
 
+# Minimal permissions for security
+permissions:
+  contents: read
+
 jobs:
   # =============================================================================
   # Backend Tests