The manual-import routes (album + singles) call post_process_matched_download
directly. When the pipeline quarantines a file — integrity / AcoustID / FLAC
bit-depth — or hits the race guard, it sets a context flag and RETURNS
NORMALLY (it only marks the task failed + notifies when there's a task_id,
which manual imports don't have). So the inner pipeline raised no exception,
and routes.py counted `processed += 1` for a file that had just been moved to
ss_quarantine, not the library. Result: the UI shows a green "Done" while the
track silently vanished — exactly the #764 report (Coldplay - Yellow.flac ->
ss_quarantine, but "Done").
The download path already handles this in
post_process_matched_download_with_verification (it reads the same flags and
marks the task failed); only the manual-import routes were missing the check.
Fix: new pure helper import_rejection_reason(context) returns a human-readable
reason for any terminal rejection (_integrity_failure_msg / _acoustid_quarantined
/ _bitdepth_rejected / _race_guard_failed) or None for a clean import. Both
manual-import routes now consult it: album_process reports the track in
`errors` instead of counting it processed; process_single_import_file returns
("error", reason) instead of ("ok", ...). Verified every move_to_quarantine
call site (4, all in pipeline.py) sets one of those flags, so no quarantine
path slips through. This also delivers the "direct display of the error" the
reporter asked for — the reason now surfaces in the response `errors` list.
Does NOT address the reverse symptom ("failed even though it moved correctly")
— not yet root-caused — nor the separate bit-depth hole on the download-path
wrapper.
Tests: tests/imports/test_import_rejection_reason.py (10) — each trigger
detected, falsy flags ignored, deterministic ordering, plus two route-level
tests driving the REAL process_single_import_file (quarantine -> "error";
clean -> "ok").
