feat(api): SHA-256 and Chromaprint fingerprints for the library (M5.5)

A background Fingerprinter runs after every scan and at startup, one file
at a time, filling media_files.sha256 and track_fingerprints (fpcalc raw,
first 120 s). Without fpcalc only SHA-256 is computed and files stay
queued for Chromaprint. Changing a file's content resets its prints.
Similarity() compares raw prints with a ±80-point alignment search; it is
the basis for duplicate detection in the upload queue.

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

services/api/internal/library/fingerprint_test.go

@@ -0,0 +1,58 @@
+package library
+
+import (
+	"math/rand/v2"
+	"testing"
+)
+
+func randomPrint(r *rand.Rand, n int) []uint32 {
+	out := make([]uint32, n)
+	for i := range out {
+		out[i] = r.Uint32()
+	}
+	return out
+}
+
+func TestSimilarity(t *testing.T) {
+	r := rand.New(rand.NewPCG(1, 2))
+	a := randomPrint(r, 950) // ~2 мин
+	if s := Similarity(a, a); s != 1 {
+		t.Fatalf("same print: %v", s)
+	}
+
+	// Перекодирование: другое вступление (сдвиг на 20 точек) и ~8% перевёрнутых бит.
+	b := append(randomPrint(r, 20), a...)
+	for i := range b {
+		for bit := range 32 {
+			if r.IntN(100) < 8 {
+				b[i] ^= 1 << bit
+			}
+		}
+	}
+	if s := Similarity(a, b); s < SameRecording {
+		t.Fatalf("re-encoded copy: %v < %v", s, SameRecording)
+	}
+
+	if s := Similarity(a, randomPrint(r, 950)); s >= SameRecording {
+		t.Fatalf("different recordings: %v >= %v", s, SameRecording)
+	}
+	if s := Similarity(a, a[:40]); s != 0 {
+		t.Fatalf("too short overlap must not match: %v", s)
+	}
+}
+
+func TestParseFPCalc(t *testing.T) {
+	fp, err := parseFPCalc([]byte(`{"duration": 215.4, "fingerprint": [4294967295, 1, 2]}`))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fp.DurationMs != 215400 || len(fp.Points) != 3 || fp.Points[0] != 4294967295 {
+		t.Fatalf("parsed %+v", fp)
+	}
+	if fp.Prefix() != int64(-4294967295) { // 0xFFFFFFFF_00000001 как int64
+		t.Fatalf("prefix %d", fp.Prefix())
+	}
+	if _, err := parseFPCalc([]byte(`{"duration": 1, "fingerprint": []}`)); err == nil {
+		t.Fatal("empty fingerprint must fail")
+	}
+}