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>

backend/init_db.py

@@ -0,0 +1,27 @@
+from sqlalchemy.orm import sessionmaker
+
+from backend.models import Base, User
+from backend.database import engine
+from backend.security import get_password_hash
+
+
+def init_db():
+    Base.metadata.create_all(bind=engine)
+
+    Session = sessionmaker(bind=engine)
+    db = Session()
+    try:
+        if not db.query(User).filter(User.username == 'admin').first():
+            db.add(User(
+                username='admin',
+                password_hash=get_password_hash('admin'),
+                role='admin',
+            ))
+            db.commit()
+            print("Created default admin user (login: admin / password: admin)")
+    finally:
+        db.close()
+
+
+if __name__ == "__main__":
+    init_db()