Initial commit: playlink music library hardlink deduplicator

Go backend (chi, SQLite) with an embedded React/TypeScript UI that finds
songs present in both a host and a slave music library and replaces the
slave copy with a hardlink to the host copy.

- Path-first matching with format/title/album-folder fallbacks
- Auto-linking of byte-identical files, re-verified with cmp before linking
- Manual review with ffprobe quality comparison, downgrade guard and
  promote-to-host override
- Build mode to populate an empty slave library from the host
- Dry run by default, atomic stage+rename replacement, quarantine with
  retention, undo, path confinement and authenticated UI
- Dockerfile and compose file for Unraid

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

internal/probe/probe_test.go

@@ -0,0 +1,57 @@
+package probe
+
+import (
+	"context"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+func TestScoreOrdering(t *testing.T) {
+	flac24 := Info{Codec: "flac", Lossless: true, BitDepth: 24, SampleRate: 96000}
+	flac16 := Info{Codec: "flac", Lossless: true, BitDepth: 16, SampleRate: 44100}
+	opus160 := Info{Codec: "opus", Bitrate: 160_000}
+	mp3320 := Info{Codec: "mp3", Bitrate: 320_000}
+	mp3128 := Info{Codec: "mp3", Bitrate: 128_000}
+	order := []Info{flac24, flac16, mp3320, opus160, mp3128}
+	for i := 1; i < len(order); i++ {
+		if Score(order[i-1]) <= Score(order[i]) {
+			t.Errorf("%v should outrank %v", order[i-1], order[i])
+		}
+	}
+	if v := Compare(mp3128, flac16); v.Better != "slave" {
+		t.Errorf("lossless slave should win, got %s", v.Better)
+	}
+	if v := Compare(mp3320, Info{Codec: "mp3", Bitrate: 310_000}); v.Better != "equal" {
+		t.Errorf("within 5%% should be equal, got %s", v.Better)
+	}
+}
+
+func TestProbeRealFiles(t *testing.T) {
+	ffmpeg, err1 := exec.LookPath("ffmpeg")
+	ffprobe, err2 := exec.LookPath("ffprobe")
+	if err1 != nil || err2 != nil {
+		t.Skip("ffmpeg/ffprobe not installed")
+	}
+	dir := t.TempDir()
+	flac, mp3 := filepath.Join(dir, "a.flac"), filepath.Join(dir, "a.mp3")
+	for _, args := range [][]string{
+		{"-f", "lavfi", "-i", "sine=f=440:d=2", "-sample_fmt", "s16", flac},
+		{"-f", "lavfi", "-i", "sine=f=440:d=2", "-b:a", "128k", mp3},
+	} {
+		if out, err := exec.Command(ffmpeg, append([]string{"-v", "error", "-y"}, args...)...).CombinedOutput(); err != nil {
+			t.Fatalf("ffmpeg: %v %s", err, out)
+		}
+	}
+	p := Prober{Bin: ffprobe}
+	fi, mi := p.Probe(context.Background(), flac), p.Probe(context.Background(), mp3)
+	if fi.Error != "" || mi.Error != "" {
+		t.Fatalf("probe errors: %q %q", fi.Error, mi.Error)
+	}
+	if !fi.Lossless || fi.BitDepth != 16 || mi.Lossless || mi.Codec != "mp3" {
+		t.Fatalf("unexpected probe results %+v / %+v", fi, mi)
+	}
+	if v := Compare(mi, fi); v.Better != "slave" {
+		t.Fatalf("flac slave should beat mp3 host, got %+v", v)
+	}
+}