Add GITEA_AUTH_HEADER env var to deployment

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

templates/backstage-deployment.yaml

@@ -102,6 +102,11 @@ spec:
             secretKeyRef:
               name: backstage-secrets
               key: GITEA_PASSWORD
+        - name: GITEA_AUTH_HEADER
+          valueFrom:
+            secretKeyRef:
+              name: backstage-secrets
+              key: GITEA_AUTH_HEADER
         # Override auth environment to production
         - name: AUTH_ENVIRONMENT
           value: "production"