Add Docker Compose setup

Multi-stage Dockerfile builds the React frontend then serves everything
from a single Python container. settings.json persists via a named volume.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/storage.py

@@ -1,9 +1,10 @@
 import json
+import os
 import secrets
 from datetime import datetime, timezone
 from pathlib import Path
 
-SETTINGS_PATH = Path(__file__).parent.parent / "settings.json"
+SETTINGS_PATH = Path(os.environ.get("SETTINGS_PATH", Path(__file__).parent.parent / "settings.json"))
 
 
 def _now():