feat(download): fill in the tracks a nearly complete album is missing

A grab that delivers nine of twelve tracks clears the completeness floor
and is imported, and the other three were never looked for. On Soulseek
that is the commonest way an album ends up almost right: one peer's
folder lacks a track, or one file fails.

After a successful import the manager now compares the tracks the files
were aligned to (ImportResult.Matched, new) with the expected tracklist.
When one to three are missing, and fewer than half, it searches for each
one on its own, as the track's artist and title with the album kept for
ranking. It grabs the first auto-acceptable copy that is not from the
source that already failed to supply it, and is not a delegate, which
would place it in its own library. The candidate is trimmed to the one
file aligned to the track. The import uses the album's own request with
ImportOptions.Only, which skips the completeness check and imports only
a file aligned to the missing track, so it is tagged and placed as part
of the album, and anything else the folder brought is left out.

One attempt per track, and nothing here fails the download: the album
is already imported, so a track that cannot be found is logged on the
job and left. slskd accepts one-file folders for a request that expects
one track, which the per-track search needs.

Closes #276

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017HJiuc3ZZhxsPXz3ozTirT

backend/download/manager.go

@@ -747,8 +747,9 @@ func (m *Manager) grab(
 	var failed []Candidate
 
 	for {
-		out := m.attemptGrab(ctx, dl, c, job)
+		out := m.attemptGrab(ctx, dl, c, job, nil)
 		if out.err == nil {
+			m.fillIn(ctx, dl, c, out.imported, job)
 			m.finishGrab(ctx, dl, out.item, out.imported, job)
 
 			return
@@ -836,6 +837,7 @@ func (m *Manager) attemptGrab(
 	dl Download,
 	c Candidate,
 	job *jobs.Handle,
+	only []ExpectedTrack,
 ) grabOutcome {
 	// Who will move the bytes is decided before any slot is taken, so
 	// the transfer waits in its own provider's queue rather than in a
@@ -942,6 +944,7 @@ func (m *Manager) attemptGrab(
 
 	opts := m.importOptions()
 	opts.WriteTags = true
+	opts.Only = only
 
 	opts.LibraryRoot, err = m.library.LibraryPath(dl.LibraryID)
 	if err != nil {