feat: add recommendation service adapters

backend/app/domain/models.py

@@ -25,6 +25,24 @@ class TrackCandidate:
     artist: str
 
 
+@dataclass(frozen=True)
+class ConversationTurn:
+    """One bounded user or assistant message supplied by the client."""
+
+    role: str
+    content: str
+
+
+@dataclass(frozen=True)
+class PreviousRecommendation:
+    """One earlier recommendation available to a refinement request."""
+
+    rank: int
+    track_id: str
+    title: str
+    artists: tuple[str, ...]
+
+
 class Familiarity(StrEnum):
     """How strongly a request should favor known or unknown music."""