ci: build and tag :beta images on beta branch pushes

Trigger CI on beta branch pushes and let build-and-push run for refs/heads/beta. The existing docker/metadata-action's type=ref,event=branch already maps the branch name into the image tag, so pushes to beta will publish ghcr.io/.../{backend,frontend}:beta automatically.

https://claude.ai/code/session_012rsJRUue9w7sBD3J5gmKWK

Author: claude

.github/workflows/ci.yml

@@ -2,7 +2,7 @@ name: CI/CD
 
 on:
   push:
-    branches: [ main ]
+    branches: [ main, beta ]
     tags: ['v*']
     paths-ignore:
       - '**.md'
@@ -126,7 +126,7 @@ jobs:
     if: |
       always() &&
       github.event_name != 'pull_request' &&
-      (github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v')) &&
+      (github.ref == 'refs/heads/main' || github.ref == 'refs/heads/beta' || startsWith(github.ref, 'refs/tags/v')) &&
       (startsWith(github.ref, 'refs/tags/v') ||
         needs.changes.outputs.backend == 'true' ||
         needs.changes.outputs.frontend == 'true') &&