Implement purge job and backend storage integration

[Shweta-Dhiman1]

Implement purge job, schema integration, and object storage upload. Issues 6, 10 and 11.
This PR implements the backend functionality for three core requirements:

1. Automated 30-Day Purge Job

• Removes student metadata (name and student ID) 30 days after assignment deadline.
• Retains anonymized source code and submission records.

2. Database Schema Integration

• Utilizes existing schema for assignments, submissions, stored_files, etc.
• Ensures compliance with constraints (e.g., language field values).

3. Object Storage Integration (MinIO)

• Added ObjectStorage helper class for file upload/download.
• Supports .java, .cpp, and .zip file handling.
• Implemented upload API endpoint to enable backend interaction with storage.

Note:

• HTTPS in transit is supported at deployment level via secure configuration.
• Files are not encrypted at rest as per project requirements.