fix: the duplicate resolver waits for a running scan, so a copy it removes is not marked present again (#6169)

At boot the scan lists the library, the resolver removes wrong-slot copies
and marks their tracks missing, then the scan reconciles from its list and
marks them present: 7 of the 13 removed on 2026-10-20 15:00 read as
present with no file, and would not be wanted back until a later scan. A
pass now waits while a scan younger than StuckScanThreshold is in flight
and tries again a minute later; a stuck run does not hold it up.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/duplicate_resolve_test.go

@@ -2,6 +2,8 @@ package library
 
 import (
 	"context"
+	"io"
+	"log/slog"
 	"os"
 	"path/filepath"
 	"testing"
@@ -549,6 +551,30 @@ func TestResolveDuplicates_AgreeingLookupsDecideAnAcousticGroup_Integration(t *t
 	})
 }
 
+// A pass while a scan is walking the library waits for it: a copy removed
+// under the walk is marked present again when the scan reconciles. A run
+// stuck past StuckScanThreshold does not hold the resolver up.
+func TestDuplicateResolveWorker_WaitsForARunningScan_Integration(t *testing.T) {
+	f := seedTrimmedAlbum(t)
+	ctx := context.Background()
+	w := NewDuplicateResolveWorker(f.pool, slog.New(slog.NewTextHandler(io.Discard, nil)), "", nil, ResolveSources{})
+	if _, err := f.pool.Exec(ctx, `INSERT INTO scan_runs DEFAULT VALUES`); err != nil {
+		t.Fatal(err)
+	}
+	if !w.tickOnce(ctx) {
+		t.Error("a pass ran under a running scan")
+	}
+	if !exists(f.aPath) || !exists(f.bPath) {
+		t.Error("a copy was removed while the scan was walking the library")
+	}
+	if _, err := f.pool.Exec(ctx, `UPDATE scan_runs SET started_at = now() - interval '2 hours'`); err != nil {
+		t.Fatal(err)
+	}
+	if w.tickOnce(ctx) {
+		t.Error("a scan stuck for two hours held the resolver up")
+	}
+}
+
 // AcoustID knows the recording as no song of the album: every copy goes. With
 // AcoustID unreachable, nothing is decided and the group waits.
 func TestResolveDuplicates_TrimmedCopiesAcoustIDCannotName_Integration(t *testing.T) {