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>

docker-compose.yml

@@ -0,0 +1,14 @@
+services:
+  app:
+    build: .
+    ports:
+      - "8000:8000"
+    volumes:
+      - data:/app/data
+    environment:
+      SETTINGS_PATH: /app/data/settings.json
+      OPENAI_MODEL: gpt-4.1
+    restart: unless-stopped
+
+volumes:
+  data: