Walks the downloads tree and reports files that fail magic-byte
validation — the "how many pre-existing files in my library are
silently truncated?" question, answered. Skips the _quarantine
subtree (already known-bad) and caps suspect_paths at 500 with a
truncated flag so the persisted report can't grow unbounded.
On-demand only: a full filesystem walk on a NAS-mounted library is
expensive and the user should choose when to pay for it. Pre-existing
files are reported, not quarantined — they may be the only copy and
the user decides what to do.
Adds GET /api/settings/library-validation (most recent report) and
POST /api/settings/library-validation/run (queue a fresh sweep).
Cached under cache.library_validation_report.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
bvandeusen