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>

Dockerfile

@@ -0,0 +1,15 @@
+FROM python:3.11-slim
+
+COPY requirements.txt /app_src/requirements.txt
+RUN pip install --no-cache-dir -r /app_src/requirements.txt
+
+COPY backend/ /app_src/backend/
+COPY frontend/ /app_src/frontend/
+COPY docker-entrypoint.sh /app_src/docker-entrypoint.sh
+RUN chmod +x /app_src/docker-entrypoint.sh
+
+WORKDIR /app
+
+EXPOSE 8000
+
+ENTRYPOINT ["/app_src/docker-entrypoint.sh"]