ci(concurrency): make cancel-in-progress event-aware, group by sha on push

Yesterday two PRs (#9724 llama.cpp bump, #9731 llama-cpp-darwin
consolidation) merged 11 seconds apart. Both shared the same
backend.yml concurrency group (ci-backends-refs/heads/master-...) due
to "${{ github.head_ref || github.ref }}" — empty head_ref on push
events falls through to the static refs/heads/master. With
cancel-in-progress: true that meant the second merge cancelled the
first's in-flight backend builds. The first PR's CI never finished;
the second PR only touched CI files so its run was a no-op.

Two changes per workflow:
- group: replace "${{ github.head_ref || github.ref }}" with
  "${{ github.event.pull_request.number || github.sha }}". On PRs
  this groups by PR number (same as before, just keyed on number not
  branch name); on push events it groups per-commit, so two master
  pushes never share a group.
- cancel-in-progress: gate on github.event_name == 'pull_request' so
  rapid pushes to a PR still cancel old runs (newer push wins) but
  master pushes never cancel each other.

Trade-off vs alternatives:
- Merge queue would also solve this and additionally test the merged
  commit before it lands. Heavier process change; out of scope here.
- Allowing per-commit master concurrency means two simultaneous master
  runs may overlap and race on tag pushes, but each commit's manifest
  digest is unique and the registry is last-writer-wins on tags —
  newer commit's tag overwrites older.

Applied to 11 workflows that share the same concurrency pattern:
backend.yml, backend_pr.yml, image.yml, image-pr.yml, lint.yml,
test.yml, test-extra.yml, tests-e2e.yml, tests-aio.yml,
tests-ui-e2e.yml, generate_intel_image.yaml.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>