Add Gitea proxy endpoint for values file fetching

Adds /gitea/api proxy to access Gitea API for fetching Helm values files
in the environment provisioner.

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

templates/backstage-configmap.yaml

@@ -81,6 +81,13 @@ data:
           changeOrigin: true
           secure: true
 
+        '/gitea/api':
+          target: '{{ .Values.integrations.gitea.url }}'
+          changeOrigin: true
+          secure: true
+          headers:
+            Authorization: 'Basic ${GITEA_AUTH_HEADER}'
+
     techdocs:
       builder: 'local'
       generator: