feat: scaffold FastAPI backend with ports-and-adapters layout

backend/app/domain/models.py

@@ -0,0 +1,30 @@
+"""Core domain models — pure data, no app-level imports."""
+
+from pydantic import BaseModel
+
+
+class Track(BaseModel):
+    """A resolved, verified track from the catalog."""
+
+    spotify_id: str
+    title: str
+    artists: list[str]
+    album: str
+    album_art_url: str | None = None
+
+
+class TasteProfile(BaseModel):
+    """Compressed listening profile feeding both LLM calls."""
+
+    top_artists_long_term: list[str]
+    top_artists_short_term: list[str]
+    top_tracks_short_term: list[str]
+    saved_track_ids: set[str]
+
+
+class Recommendation(BaseModel):
+    """A ranked track with its one-line justification."""
+
+    track: Track
+    justification: str
+    rank: int