`_scan_all_source_roots_async` unconditionally dispatched backfill_gps
30s after each container boot. backfill_gps then queued one
extract_metadata task for every photo where latitude IS NULL — which is
most of the library (screenshots, indoor shots, scans, anything without
GPS in EXIF). The result was ~60k extract_metadata tasks piling onto
the default queue at every deploy, pinning worker-light at 180+% CPU
for ~30 min while it re-derived metadata that wasn't going to change.
The "scanned-before-the-GPS-fix" rationale in the original comment
hasn't applied for many releases. Manual trigger via
POST /api/v1/library/backfill-gps is preserved for the rare case where
the extractor really did change.
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