feat: add key-free demo mode replaying recorded fixtures

backend/app/adapters/demo/playlist.py

@@ -0,0 +1,28 @@
+"""Playlist writer that makes demo saves explicit and network-free."""
+
+import hashlib
+
+import structlog
+
+from app.domain.models import CreatedPlaylist
+
+
+class DemoPlaylistWriter:
+    """Simulate playlist writes with stable Spotify-shaped URLs."""
+
+    async def create_playlist(self, name: str, description: str) -> CreatedPlaylist:
+        """Return a deterministic fake playlist without an external write."""
+        digest = hashlib.sha256(f"{name}\n{description}".encode()).hexdigest()[:12]
+        playlist_id = f"demo-{digest}"
+        return CreatedPlaylist(
+            id=playlist_id,
+            url=f"https://open.spotify.com/playlist/{playlist_id}",
+        )
+
+    async def add_tracks_to_playlist(self, playlist_id: str, track_uris: list[str]) -> None:
+        """Log the simulated track addition without writing to Spotify."""
+        structlog.get_logger().info(
+            "demo_playlist_simulated",
+            playlist_id=playlist_id,
+            track_count=len(track_uris),
+        )