feat: add Spotify authentication and sessions

backend/app/domain/models.py

@@ -0,0 +1,16 @@
+"""Pure domain models shared across application boundaries."""
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class Track:
+    """A Spotify track expressed without transport-specific data."""
+
+    id: str
+    uri: str
+    title: str
+    artists: tuple[str, ...]
+    album_name: str
+    album_art_url: str | None
+    external_url: str | None