Add fallback logic to find-upstream-image for timing races

When a squash merge lands on develop, the new commit's image may not
exist yet (pipeline still building). Previously, downstream repos would
fail immediately when the latest commit's image wasn't found.

Now the script iterates through recent source commits (up to 10 by
default) and uses the first one that has an available image. This
prevents failures due to timing races between upstream and downstream
pipelines.

Changes:
- find-upstream-image.sh: Check multiple commits, use first with image
- Add --max-search option to control fallback depth (default: 10)
- Add UPSTREAM_FALLBACK output variable to indicate when fallback used
- Add UPSTREAM_OVERRIDE_TAG variable to skip lookup and use specific tag
- Add UPSTREAM_MAX_SEARCH variable to template

Fixes balance_tracker pipeline failures when HAF develop is updated
but image isn't built yet.