Scripts: Add cleanup tool for expired internal submission S3 artifacts

[RishabhJain2018]

Summary

• Add cleanup_expired_internal_submissions.py to delete S3 submission artifact files older than 14 days on internal challenges while preserving DB Submission rows.
• Reuse existing S3 cleanup helpers from s3.py and artifact path discovery from jobs.s3_retention.
• Cross-reference the new script from the existing challenge-wide S3 cleanup tool docstring.

Test plan

• Run dry run: python scripts/tools/cleanup_data/cleanup_expired_internal_submissions.py
• Verify only internal, non-host-credential challenges are included and cutoff uses submitted_at
• Run scoped dry run with --challenge-ids on a known internal challenge
• On staging, run --execute --challenge-ids <pilot> and confirm S3 objects are removed while DB rows remain

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.72%. Comparing base (7d19d31) to head (b3867bf).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5092   +/-   ##
=======================================
  Coverage   91.72%   91.72%           
=======================================
  Files          96       96           
  Lines        8155     8155           
=======================================
  Hits         7480     7480           
  Misses        675      675           

Flag	Coverage Δ
backend	95.20% <ø> (ø)
frontend	87.44% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Accounts & Authentication	97.31% <ø> (ø)
Challenges Management	95.52% <ø> (ø)
Job Processing	89.79% <ø> (ø)
Participants & Teams	100.00% <ø> (ø)
Challenge Hosts	100.00% <ø> (ø)
Analytics	100.00% <ø> (ø)
Web Interface	100.00% <ø> (ø)
Frontend (Gulp)	87.44% <ø> (ø)
All Models	97.80% <ø> (ø)
All Views	100.00% <ø> (ø)
All Serializers	98.60% <ø> (ø)
Utility Functions	96.67% <ø> (ø)
Core Configuration	82.35% <ø> (ø)

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