ramonskie's thread finding: opening the Import page fires staging files + groups + hints
together, and each one independently os.walk'd the whole staging folder AND mutagen-read every
file's tags — 3x the directory walk and 3x the per-file tag I/O on every page open (the import
scan storm + memory spike on large staging folders).
they all need the same per-file tag data, so scan ONCE: _scan_staging_records walks staging and
reads each file's metadata a single time, returning per-file records that files/groups/hints all
derive from in-memory. a short TTL (6s) + a lock means the three near-simultaneous page-open
requests share one scan instead of each kicking off a full re-read; the lock also prevents
concurrent full scans. hints now derives from the same read_staging_file_metadata the other two
use (same underlying tags) instead of a separate read_tags pass. album/singles process drop the
cache on completion so the list updates immediately after files leave staging.
net: 1 walk + 1 tag-read-per-file on page open instead of 3. 2 tests (shared-scan: 3 endpoints =
1 read per file, not 3; hints updated to the shared reader) + autouse cache-clear fixture; 695
import/staging tests green.
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