merge: integrate upstream v0.1.94 into release/custom-0.1.94

Merge upstream v0.1.94 onto our custom branch (based on release/custom-0.1.93).

Key additions beyond upstream:
- Startup stale concurrency slot cleanup on process restart
- iframe language passthrough for embedded payment/custom pages
- embedded-url.spec.ts frontend tests
- Admin payment integration API docs update

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

.github/workflows/backend-ci.yml

@@ -44,6 +44,6 @@ jobs:
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v9
         with:
-          version: v2.11
+          version: v2.9
           args: --timeout=30m
           working-directory: backend