feat(qobuz): parse the v10 embedded lyrics translation block

QobuzLyricsDocument gains the optional translation member (same content
union as original, new in API 10.0), the content DTOs become
Serialize+PartialEq so the lyrics cache can persist a translation
verbatim, and a lang() accessor exposes the content language. New
fixtures cover embedded translations of each type (wsync/lsync/plain),
the lyricsUrl envelope with translation_requested, empty
translation_langs (feature off), and 9.x documents (no translation).

km

crates/qbz-qobuz/tests/fixtures/lyrics-doc-plain-translation.json

@@ -0,0 +1,23 @@
+{
+  "album_id": "def456",
+  "track_id": 222222222,
+  "translation_langs": ["de"],
+  "publishers": [],
+  "writers": null,
+  "original": {
+    "type": "plain",
+    "lang": "en",
+    "lines": [
+      {"line": "Plain original one"},
+      {"line": "Plain original two"}
+    ]
+  },
+  "translation": {
+    "type": "plain",
+    "lang": "de",
+    "lines": [
+      {"line": "Einfache Uebersetzung eins"},
+      {"line": "Einfache Uebersetzung zwei"}
+    ]
+  }
+}