implement native Deezer download/decrypt pipeline

Replace Deezer yt-dlp usage with native ARL session + media.get_url resolution, add BF_CBC_STRIPE decryption in downloader, and wire cipher-aware Deezer downloads through the main rip pipeline. Includes validation hardening and metadata/source-id improvements used by tagging flows.

internal/download/downloader_test.go

@@ -2,11 +2,14 @@ package download
 
 import (
 	"context"
+	"crypto/cipher"
 	"net/http"
 	"net/http/httptest"
 	"os"
 	"path/filepath"
 	"testing"
+
+	"golang.org/x/crypto/blowfish"
 )
 
 func TestDownloaderHasNoClientTimeout(t *testing.T) {
@@ -51,3 +54,33 @@ func TestManifestDetection(t *testing.T) {
 		t.Fatalf("did not expect flac to be manifest")
 	}
 }
+
+func TestNormalizeDeezerTrackID(t *testing.T) {
+	if got := normalizeDeezerTrackID("https://www.deezer.com/track/3135556"); got != "3135556" {
+		t.Fatalf("normalize track id = %q, want 3135556", got)
+	}
+}
+
+func TestDecryptDeezerBFCBCStripe(t *testing.T) {
+	trackID := "3135556"
+	plain := make([]byte, deezerBFChunkSize*2)
+	for i := range plain {
+		plain[i] = byte(i % 251)
+	}
+	enc := make([]byte, len(plain))
+	copy(enc, plain)
+	block, err := blowfish.NewCipher(deriveDeezerBlowfishKey(trackID))
+	if err != nil {
+		t.Fatalf("cipher error: %v", err)
+	}
+	cbc := cipher.NewCBCEncrypter(block, deezerBFIV)
+	cbc.CryptBlocks(enc[:deezerBFChunkSize], enc[:deezerBFChunkSize])
+
+	dec, err := decryptDeezerBFCBCStripe(enc, trackID)
+	if err != nil {
+		t.Fatalf("decrypt error: %v", err)
+	}
+	if len(dec) != len(plain) || string(dec) != string(plain) {
+		t.Fatalf("decrypted data mismatch")
+	}
+}