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()

backend/app/main.py

@@ -0,0 +1,27 @@
+"""Application factory and wiring. No logic lives here."""
+
+from pathlib import Path
+
+from fastapi import FastAPI
+from fastapi.staticfiles import StaticFiles
+
+from app.config import settings
+
+FRONTEND_DIST = Path(__file__).parent / "static"
+
+
+def create_app() -> FastAPI:
+    """Build the FastAPI app: API routes plus the built SPA on one port."""
+    app = FastAPI(title="discovery-by-llm", docs_url=None, redoc_url=None)
+
+    @app.get("/api/health")
+    def health() -> dict[str, str]:
+        return {"status": "ok", "mode": settings.app_mode}
+
+    if FRONTEND_DIST.is_dir():
+        app.mount("/", StaticFiles(directory=FRONTEND_DIST, html=True), name="spa")
+
+    return app
+
+
+app = create_app()