feat(discovery): SimilaritySource protocol + FakeSimilaritySource

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/lyra_worker/similarity/base.py

@@ -0,0 +1,22 @@
+from dataclasses import dataclass
+from typing import Protocol, runtime_checkable
+
+
+@dataclass(frozen=True)
+class SimilarArtist:
+    mbid: str
+    name: str
+    score: float
+
+
+@runtime_checkable
+class SimilaritySource(Protocol):
+    name: str
+
+    def health(self) -> bool:
+        """Is this source reachable right now?"""
+        ...
+
+    def similar_artists(self, mbid: str) -> list[SimilarArtist]:
+        """Artists similar to the given artist MBID (MBID-native)."""
+        ...