Allow zero submission rate limits

[msaroufim]

Summary

• allow admin rate limits to use 0 submissions/hour
• add a migration relaxing the DB constraint from > 0 to >= 0
• cover zero-limit behavior in DB tests so a visible leaderboard can reject every new submission

Validation

• uv run --extra dev pytest -q tests/test_admin_api.py::TestAdminRateLimits tests/test_leaderboard_db.py -k 'rate_limit'
• python3 -m py_compile src/kernelbot/api/main.py src/libkernelbot/leaderboard_db.py src/migrations/20260319_01_allow-zero-rate-limits.py

Operational note

After this is deployed, set qr to 0 submissions/hour for both test and leaderboard categories to keep it visible while blocking new submissions.

[github-actions]

Coverage report

Click to see where and how coverage changed

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

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