Add missing-only validation continuation

[msaroufim]

Summary

• add --only-missing to manual application-validation sweeps
• skip exact submissions that already have a result for the current GPU and contract
• preserve all-users selection, nightly top-10 behavior, and two-job concurrency
• add database, service, API, CLI, and documentation coverage

Why

The 93-user Cholesky backfill completed successfully, but three users submitted new best kernels during the 1h13m sweep. A missing-only continuation validates those exact current submissions without rerunning the 90 unchanged kernels.

Kernelboard requires no update because it already joins results by exact submission, GPU, and contract version.

Validation

• focused database/service/API/CLI suite — 13 passed
• Ruff on all changed Python files
• git diff --check

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/libkernelbot
application_validation.py					108, 317
leaderboard_db.py					958, 970
utils.py
Project Total

This report was generated by python-coverage-comment-action