Add admin bulk submission delete endpoint

[msaroufim]

Summary

• add an admin API to delete all submissions for a given leaderboard ID and username
• delete dependent submission job status rows and runs before deleting submissions
• add API and DB tests covering successful deletion and missing leaderboard handling

Validation

• uv run --extra dev pytest tests/test_admin_api.py -q
• uv run --extra dev pytest tests/test_leaderboard_db.py -q
• exercised the endpoint locally against a temporary embedded Postgres instance with migrations applied; verified target submissions, runs, and submission_job_status rows were removed while another user's submission remained

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/libkernelbot
leaderboard_db.py
utils.py
Project Total

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