fix(gallery): add fallback URI resolution for backend installation (#8663)

* fix(gallery): add fallback URI resolution for backend installation

When a backend installation fails (e.g., due to missing 'latest-' tag),
try fallback URIs in order:
1. Replace 'latest-' with 'master-' in the URI
2. If that fails, append '-development' to the backend name

This fixes the issue where backend index entries don't match the
repository tags. For example, installing 'ace-step' tries to download
'latest-gpu-nvidia-cuda-13-ace-step' but only 'master-gpu-nvidia-cuda-13-ace-step'
exists in the quay.io registry.

Fixes: #8437
Signed-off-by: localai-bot <139863280+localai-bot@users.noreply.github.com>

* chore(gallery): make fallback URI patterns configurable via env vars

---------

Signed-off-by: localai-bot <139863280+localai-bot@users.noreply.github.com>