Adds a second borgmatic config (photos.yaml) that backs up /Volumes/photos
(sifaka SMB mount) to a dedicated BorgBase repo, running daily at 4 AM.
Refactors borgmatic metrics to support multiple repos with a `repo` label,
and updates the Grafana dashboard with a repo selector variable.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>