Hackathon Submissions API

[InfoX1337]

Dependencies: #10

Scope: Implement CRUD for the HackathonTaskSubmission model.

Validation: Enforce transitions between DRAFT and ACTIVE statuses. Reject updates or final submissions if the current server time exceeds Task.submissions_end_at. Integrate with the Media Store to accept attachments.

[olix3001]

Use object storage for uploads like images, but require url for videos!