Implemented shared S3 audio caching for prepare and restore --include-audio

internal/artifacts/paths_model_test.go

@@ -114,3 +114,46 @@ func TestSessionSpoolAudioDir(t *testing.T) {
 		t.Fatalf("SessionSpoolAudioDir() = %q, want %q", got, want)
 	}
 }
+
+func TestSessionSpoolRestoreAudioDir(t *testing.T) {
+	root := "/var/spool/narratio"
+	got := SessionSpoolRestoreAudioDir(root, "forsaken", "2026-04-19")
+	want := filepath.Join(root, "forsaken", "2026-04-19", "restore", "audio")
+	if got != want {
+		t.Fatalf("SessionSpoolRestoreAudioDir() = %q, want %q", got, want)
+	}
+}
+
+func TestS3AudioCachePath(t *testing.T) {
+	got, err := S3AudioCachePath("/var/cache/narratio", "my-dnd-archive", "dnd/campaigns/forsaken/sessions/2026-04-19/audio/alice.flac")
+	if err != nil {
+		t.Fatalf("S3AudioCachePath() error = %v", err)
+	}
+	want := filepath.Join("/var/cache/narratio", "s3", "my-dnd-archive", "dnd", "campaigns", "forsaken", "sessions", "2026-04-19", "audio", "alice.flac")
+	if got != want {
+		t.Fatalf("S3AudioCachePath() = %q, want %q", got, want)
+	}
+}
+
+func TestS3AudioCachePathRejectsUnsafeInputs(t *testing.T) {
+	tests := []struct {
+		name   string
+		root   string
+		bucket string
+		key    string
+	}{
+		{name: "empty root", root: "", bucket: "bucket", key: "audio/a.flac"},
+		{name: "empty bucket", root: "/cache", bucket: "", key: "audio/a.flac"},
+		{name: "bucket slash", root: "/cache", bucket: "bad/bucket", key: "audio/a.flac"},
+		{name: "empty key", root: "/cache", bucket: "bucket", key: ""},
+		{name: "escaping key", root: "/cache", bucket: "bucket", key: "../audio/a.flac"},
+		{name: "absolute key", root: "/cache", bucket: "bucket", key: "/audio/a.flac"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got, err := S3AudioCachePath(tt.root, tt.bucket, tt.key); err == nil {
+				t.Fatalf("S3AudioCachePath() = %q, want error", got)
+			}
+		})
+	}
+}