fix: a track shorter than the fingerprint span keeps its print (#6178)

The image's fpcalc 1.5.1 exits 3 with "Error decoding audio frame (End
of file)" whenever it reaches the end of a file before -length, at any
length, so every track under two minutes failed both the AcoustID lookup
print and the duplicate-detection print: 471 lookups and all 1,979
present short tracks' chromaprints.

- runFingerprintTool returns the tool's stdout beside a toolExitError.
- fpcalcReadToEnd keeps a partial exit whose stderr is the end of file
  and whose declared DURATION is within the span asked for. A damaged
  file that stops early declares more than it read, and still fails.
- Migration 0097 clears the stored failures so the lookup worker and
  the fingerprint backfill take those tracks again.

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

internal/library/fingerprint_test.go

@@ -253,3 +253,45 @@ func TestFingerprintResult_InconclusiveIfEitherHalfIs(t *testing.T) {
 		}
 	}
 }
+
+// fpcalc 1.5.1 exits 3 with "(End of file)" on every file shorter than the
+// span asked for (#6178). That print covers the whole file and is kept; a file
+// whose declared length runs past the span stopped early, and is not.
+func TestFpcalcReadToEnd(t *testing.T) {
+	eof := []byte("ERROR: Error decoding audio frame (End of file)")
+	out := func(sec int) []byte { return []byte(fmt.Sprintf("DURATION=%d\nFINGERPRINT=AQAB\n", sec)) }
+	for name, tc := range map[string]struct {
+		err  error
+		out  []byte
+		want bool
+	}{
+		"short file read to its end":   {&toolExitError{name: "fpcalc", code: 3, stderr: eof}, out(37), true},
+		"exactly the span":             {&toolExitError{name: "fpcalc", code: 3, stderr: eof}, out(120), true},
+		"declared longer than read":    {&toolExitError{name: "fpcalc", code: 3, stderr: eof}, out(150), false},
+		"partial for another reason":   {&toolExitError{name: "fpcalc", code: 3, stderr: []byte("ERROR: invalid data")}, out(37), false},
+		"a different exit":             {&toolExitError{name: "fpcalc", code: 2, stderr: eof}, out(37), false},
+		"no duration printed":          {&toolExitError{name: "fpcalc", code: 3, stderr: eof}, []byte("FINGERPRINT=AQAB\n"), false},
+		"not a tool exit (a stall)":    {fmt.Errorf("fpcalc: %w", errFingerprintTimeout), out(37), false},
+		"wrapped exit is still judged": {fmt.Errorf("lookup: %w", &toolExitError{name: "fpcalc", code: 3, stderr: eof}), out(37), true},
+	} {
+		if got := fpcalcReadToEnd(tc.err, tc.out, 120); got != tc.want {
+			t.Errorf("%s: fpcalcReadToEnd = %v, want %v", name, got, tc.want)
+		}
+	}
+}
+
+// The output of a tool that exited non-zero reaches the caller, or
+// fpcalcReadToEnd has nothing to read; the message is unchanged.
+func TestRunFingerprintTool_KeepsOutputOnExit(t *testing.T) {
+	out, err := runFingerprintTool(context.Background(), "sh",
+		[]string{"-c", "echo DURATION=37; echo 'ERROR: Error decoding audio frame (End of file)' >&2; exit 3"})
+	if err == nil {
+		t.Fatal("exit 3 returned no error")
+	}
+	if want := "sh exited 3: ERROR: Error decoding audio frame (End of file)"; err.Error() != want {
+		t.Errorf("err = %q, want %q", err, want)
+	}
+	if d, ok := fpcalcDuration(out); !ok || d != 37 {
+		t.Errorf("output = %q, want the DURATION line kept", out)
+	}
+}