Complete the migration of the batch service from MongoDB to SQLite using an adapter pattern:

- Create test database helper supporting both MongoDB and SQLite backends
- Update Sequelize models to use test database in test environments
- Add database initialization and cleanup in batch service tests
- Modify batch test script to use SQLite instead of MongoDB
- Install mongodb-memory-server for testing MongoDB operations
- Fix Sequelize initialization in test environment

This migration maintains full functional parity with the previous MongoDB implementation
while making the system more flexible and easier to deploy for academic/demo purposes.
All 15 batch service tests now pass with the SQLite adapter.

Environment variables:
- USE_SQLITE=true to enable SQLite backend
- SKIP_MONGODB=true to skip MongoDB connection