feat: add manual cleanup workflow for stuck builds (#277)

Adds a workflow_dispatch triggered workflow that calls the versioning
backend's cleanUpStuckBuilds endpoint. Maintainers can trigger it from
the Actions tab to process all stuck "started" builds immediately.

Requires typing "yes" to confirm, to prevent accidental triggers.
Uses the existing VERSIONING_TOKEN secret for auth.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: frostebite

.github/workflows/clean-up-stuck-builds.yml

@@ -0,0 +1,36 @@
+name: Clean Up Stuck Builds
+
+on:
+  workflow_dispatch:
+    inputs:
+      confirm:
+        description: 'Type "yes" to confirm cleanup of all stuck builds'
+        required: true
+        default: ''
+
+jobs:
+  cleanup:
+    name: Clean up stuck builds
+    runs-on: ubuntu-latest
+    if: ${{ github.event.inputs.confirm == 'yes' }}
+    steps:
+      - name: Trigger manual cleanup
+        run: |
+          echo "Triggering manual cleanup of stuck builds..."
+          RESPONSE=$(curl -s -w "\n%{http_code}" -X POST \
+            -H "Authorization: Bearer ${{ secrets.VERSIONING_TOKEN }}" \
+            -H "Content-Type: application/json" \
+            "https://europe-west3-unity-ci-versions.cloudfunctions.net/cleanUpStuckBuilds")
+
+          HTTP_CODE=$(echo "$RESPONSE" | tail -1)
+          BODY=$(echo "$RESPONSE" | head -n -1)
+
+          echo "HTTP Status: $HTTP_CODE"
+          echo "$BODY" | jq . 2>/dev/null || echo "$BODY"
+
+          if [ "$HTTP_CODE" -ne 200 ]; then
+            echo "::error::Cleanup request failed with status $HTTP_CODE"
+            exit 1
+          fi
+
+          echo "::notice::Cleanup completed successfully"