fix: use in-cluster Gitea URL for backstage proxy target

The /gitea/api proxy was pointing at the external Gitea hostname which
is unreachable from inside the Kubernetes cluster (no hairpin NAT). Switch
to the in-cluster service DNS, set secure: false for plain HTTP, and
remove the redundant 'Basic ' prefix from the Authorization header.

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

templates/backstage-configmap.yaml

@@ -104,11 +104,11 @@ data:
           secure: true
 
         '/gitea/api':
-          target: '{{ .Values.integrations.gitea.url }}'
+          target: '{{ .Values.backstage.gitea.inClusterUrl }}'
           changeOrigin: true
-          secure: true
+          secure: false
           headers:
-            Authorization: 'Basic ${GITEA_AUTH_HEADER}'
+            Authorization: ${GITEA_AUTH_HEADER}
 
         '/github/api':
           target: 'https://api.github.com'