feat: offer a demo instance link from the live deployment

backend/app/config.py

@@ -18,6 +18,9 @@ class Settings(BaseSettings):
     model_config = SettingsConfigDict(env_file=".env", extra="ignore")
 
     app_mode: AppMode = AppMode.DEMO
+    # Optional public URL of a fixture-replay twin of this deployment; the UI
+    # offers it as an escape hatch when the live Spotify quota runs out.
+    demo_instance_url: str = ""
 
     # Spotify application and OAuth. The redirect URI must exactly match a
     # URI registered in the Spotify developer dashboard, port included.

backend/app/main.py

@@ -124,7 +124,10 @@ def create_app(
 
     @app.get("/api/health")
     def health() -> dict[str, str]:
-        return {"status": "ok", "mode": active_settings.app_mode}
+        payload = {"status": "ok", "mode": active_settings.app_mode}
+        if active_settings.demo_instance_url:
+            payload["demo_url"] = active_settings.demo_instance_url
+        return payload
 
     app.include_router(router)
     app.include_router(recommendations_router)