feat: add Spotify authentication and sessions

backend/app/adapters/spotify/errors.py

@@ -0,0 +1,31 @@
+"""Typed failures raised by the Spotify adapter."""
+
+
+class SpotifyError(Exception):
+    """Base class for Spotify adapter failures."""
+
+
+class SpotifyAuthenticationError(SpotifyError):
+    """Spotify rejected authentication or token refresh."""
+
+
+class SpotifyRateLimitedError(SpotifyError):
+    """Spotify rate limited a request that could not be retried."""
+
+    def __init__(self, retry_after_seconds: float | None) -> None:
+        """Record Spotify's requested delay without exposing request data."""
+        super().__init__("Spotify rate limit exceeded")
+        self.retry_after_seconds = retry_after_seconds
+
+
+class SpotifyUnavailableError(SpotifyError):
+    """Spotify returned a server-side failure."""
+
+
+class SpotifyRequestError(SpotifyError):
+    """Spotify rejected a non-authenticated API request."""
+
+    def __init__(self, status_code: int) -> None:
+        """Record the response status without exposing response content."""
+        super().__init__(f"Spotify request failed with status {status_code}")
+        self.status_code = status_code