ci: add non-blocking ally accessibility scan

.github/workflows/ally-a11y.yml

@@ -0,0 +1,32 @@
+# Accessibility scan via ally (own axe-core a11y tool). Non-blocking/informational.
+name: Ally A11y Scan
+
+on:
+  schedule:
+    - cron: "30 8 * * 1" # Mondays 08:30 UTC
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  a11y:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Ally accessibility scan
+        continue-on-error: true
+        uses: forbiddenlink/ally@main
+        with:
+          path: "."
+          threshold: "9999"
+          max-files: "500"
+          output: ".ally"
+      - name: Upload report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: ally-a11y-report
+          path: ".ally/**"
+          if-no-files-found: warn
+          retention-days: 30