prune_missing_photos() previously only deleted Photo rows whose files
were gone, leaving every folder row from the old library in the DB —
which made the sidebar tree wildly out of sync with the on-disk
structure (still showing /photos/2024/, /photos/2026/03/, etc. that
no longer exist).
Now also drops Folder rows whose path doesn't resolve under a mounted
source root, with the same defensive "skip if source root unmounted"
guard. The Settings orphan card surfaces both counts.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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