Fix CI test: mount DEPLOY_DIR instead of subdirectory

Docker daemon can't bind-mount a subdirectory that it can't independently
verify — mount the parent and set -w /src/backend instead.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -26,8 +26,8 @@ jobs:
       - name: Test
         run: |
           docker run --rm \
-            -v ${{ vars.DEPLOY_DIR }}/backend:/app \
+            -v ${{ vars.DEPLOY_DIR }}:/src \
-            -w /app \
+            -w /src/backend \
             golang:1.25-alpine \
             go test ./...