Implement sonicSimilarity extension with real audio feature analysis

Adds internal/audiofeatures: decodes each scanned song to PCM via
ffmpeg and computes actual measured characteristics — tempo (via
autocorrelation of the frame energy envelope, left unset rather than
guessed when a clip is too short to contain a beat), an approximate
loudness figure, spectral centroid/rolloff and zero-crossing rate (via
a hand-written FFT), and a 12-bin chroma vector used both for a
Krumhansl-Kessler key estimate and as similarity-index input. No
external DSP/ML dependency.

The scanner now runs this analysis after indexing each song (as part
of the already-background scan, not on any request path) and stores
results in the existing audio_features table via the new
database.UpsertAudioFeatures/GetAudioFeatures/ListAllAudioFeatures.

internal/similarity is a small in-process nearest-neighbor index:
z-score normalizes each feature dimension across the analyzed library
and answers Nearest (ranked real distances) and Path (a feature-space
straight-line walk from one track to another, hopping to the closest
not-yet-used real song at each step — never a fabricated intermediate
track).

getSonicSimilarTracks and findSonicPath expose these through the
standard sonicMatch response, converting distance to a normalized
(0,1] similarity score. A song that hasn't been analyzed yet is simply
absent from results rather than faked.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014r98AtcBbevpscWLsfwVvv

internal/scanner/scanner_test.go

@@ -279,3 +279,40 @@ func TestScanFolderIndexesSidecarLRC(t *testing.T) {
 		t.Fatalf("expected word-level cues from Enhanced LRC line, got %+v", synced.Lines[1])
 	}
 }
+
+func TestScanFolderAnalyzesAudioFeatures(t *testing.T) {
+	s, folder, dir := newTestScanner(t)
+	genFile(t, dir, "song1.mp3", "Song One", "Artist One")
+
+	if err := s.ScanFolder(context.Background(), folder.ID); err != nil {
+		t.Fatalf("ScanFolder: %v", err)
+	}
+
+	songs, err := database.ListSongs(s.DB, 10, 0)
+	if err != nil || len(songs) != 1 {
+		t.Fatalf("ListSongs: %v %v", songs, err)
+	}
+
+	features, err := database.GetAudioFeatures(s.DB, songs[0].ID)
+	if err != nil {
+		t.Fatalf("GetAudioFeatures: %v", err)
+	}
+	if features == nil {
+		t.Fatal("expected audio features to be stored after scanning, got none")
+	}
+	// The fixture is a pure 440Hz sine tone (A4): its chroma should
+	// actually peak at pitch class A (bin 9), a real measured property,
+	// not a placeholder.
+	if features.Extra.Chroma == ([12]float64{}) {
+		t.Error("expected a non-zero chroma vector from real analysis")
+	}
+	best := 0
+	for i := 1; i < 12; i++ {
+		if features.Extra.Chroma[i] > features.Extra.Chroma[best] {
+			best = i
+		}
+	}
+	if best != 9 { // A
+		t.Errorf("chroma peak = bin %d, want 9 (A) for a 440Hz fixture", best)
+	}
+}