chore: fix pre-existing lint issues blocking commits

- wsl_v5: blank line before t.Fatal after rows.Close
- staticcheck SA5011: explicit return after t.Fatal for nil guards

No behavior change.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/metadata/flacduration_test.go

@@ -69,6 +69,8 @@ func TestGetFlacDuration_BasicParsing(t *testing.T) {
 
 			if props == nil {
 				t.Fatal("expected non-nil AudioProperties")
+
+				return
 			}
 
 			if props.SampleRate <= 0 {