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

backend/app/config.py

@@ -0,0 +1,23 @@
+"""Typed application settings: the only reader of environment variables."""
+
+from enum import StrEnum
+
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class AppMode(StrEnum):
+    """Runtime mode: live Spotify + Anthropic, or fixture-replay demo."""
+
+    LIVE = "live"
+    DEMO = "demo"
+
+
+class Settings(BaseSettings):
+    """Application settings, loaded from the environment or a .env file."""
+
+    model_config = SettingsConfigDict(env_file=".env", extra="ignore")
+
+    app_mode: AppMode = AppMode.DEMO
+
+
+settings = Settings()