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>

.dockerignore

@@ -0,0 +1,13 @@
+venv/
+backend/venv/
+**/__pycache__/
+*.pyc
+*.pyo
+*.db
+.git/
+.gitignore
+.vscode/
+.idea/
+data/
+*.xls
+*.xlsx