feat: add Docker Compose setup with bind-mount appdata directory

- Dockerfile builds image with app source in /app_src
- docker-entrypoint.sh syncs code from image on each start, inits DB
- docker-compose.yml mounts ./appdata:/app for easy backup
- backend/init_db.py for safe DB init (no drop_all)
- backend/database.py supports DATABASE_URL env var
- .dockerignore excludes venv, __pycache__, *.db, .git

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

docker-entrypoint.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+set -e
+
+# Sync app code from image on every start (keeps code fresh after rebuilds)
+rm -rf /app/backend /app/frontend
+cp -r /app_src/backend /app_src/frontend /app/
+
+mkdir -p /app/data
+python -m backend.init_db
+
+exec uvicorn backend.main:app --host 0.0.0.0 --port 8000