feat: add YouTubeAdapter with injectable client seam

worker/lyra_worker/adapters/youtube.py

@@ -0,0 +1,55 @@
+from typing import Callable, Protocol
+
+from lyra_worker.types import Candidate, DownloadResult, MBTarget, Quality
+
+_YT_QUALITY = Quality(fmt="OPUS", lossless=False, bitrate_kbps=160)
+
+
+class YtClient(Protocol):
+    def search_album(self, artist: str, album: str) -> list[dict]:
+        """Return album matches: dicts with keys source_ref, title, artist, track_count."""
+        ...
+
+    def download(self, source_ref: str, dest: str, on_progress: Callable[[float], None]) -> dict:
+        """Download into dest; return {track_count, path}. Raise on failure."""
+        ...
+
+
+class YouTubeAdapter:
+    name = "youtube"
+    tier = 2
+
+    def __init__(self, client: YtClient):
+        self._client = client
+
+    def health(self) -> bool:
+        return True
+
+    def search(self, target: MBTarget) -> list[Candidate]:
+        candidates: list[Candidate] = []
+        for r in self._client.search_album(target.artist, target.album):
+            candidates.append(
+                Candidate(
+                    source="youtube",
+                    source_ref=r["source_ref"],
+                    matched_artist=r.get("artist", ""),
+                    matched_album=r.get("title", ""),
+                    quality=_YT_QUALITY,
+                    track_count=r.get("track_count", 1) or 1,
+                    source_tier=self.tier,
+                )
+            )
+        return candidates
+
+    def download(
+        self, candidate: Candidate, dest: str, on_progress: Callable[[float], None]
+    ) -> DownloadResult:
+        try:
+            info = self._client.download(candidate.source_ref, dest, on_progress)
+        except Exception as e:  # network/extraction failures -> fall-through in the pipeline
+            return DownloadResult(ok=False, error=str(e))
+        return DownloadResult(
+            ok=True,
+            path=info.get("path", dest),
+            track_count=info.get("track_count", candidate.track_count),
+        )