ci: run the Tests workflow on stable-targeted PRs

tests.yml (the backend/frontend Jest+Vitest jobs) only triggered on
main/beta, but those two jobs are required status checks on the stable
branch. A beta→stable promote PR therefore hung forever on
'Expected — Waiting for status to be reported' for backend/frontend,
while docker-build / install-smoke / schema-drift (already listing
stable) ran fine. Add stable to the push + pull_request filters so the
Tests suite runs on promote PRs too.

.github/workflows/tests.yml

@@ -17,9 +17,9 @@ name: Tests
 
 on:
   push:
-    branches: [main, beta]
+    branches: [main, beta, stable]
   pull_request:
-    branches: [main, beta]
+    branches: [main, beta, stable]
   workflow_dispatch:
 
 permissions: