Kernelguard pre-queue check addition

[SinatrasC]

Summary

Integrates KernelGuard as a submission pre-check gate that detects and rejects exploit submissions before GPU execution. The check runs at the API boundary, before queue enqueue on the async path and before GPU dispatch on the sync path, so blocked payloads never consume worker resources. Flagged submissions are recorded with a "hacked" status in the database. The feature is off by default (KERNELGUARD_ENABLED) and supports both fail-open and fail-closed modes for handling analyzer outages.

Changes

• New: src/libkernelbot/kernelguard.py - CLI wrapper for kernelguard tool with env-var configuration
• New: tests/test_kernelguard.py - unit tests for mode gating, rejection, fail-open/closed
• Modified: backend.py - precheck in submit_full() with skip_precheck flag, submission_started guard
• Modified: main.py - precheck before enqueue_background_job() in async endpoint
• Modified: api_utils.py - catch KernelGuardRejected (400) and KernelBotError (503) separately
• Modified: background_submission_manager.py - handle KernelGuardRejected with "hacked" job status
• Modified: leaderboard_db.py - mark_submission_hacked() method
• Modified: pyproject.toml - add kernelguard>=0.1.1, bump requires-python to >=3.11

Environment Variables

Variable	Purpose	Default
KERNELGUARD_ENABLED	Master switch	off
KERNELGUARD_TIMEOUT_SEC	CLI subprocess timeout	30s
KERNELGUARD_PROFILE	Built-in profile (default/strict/generic)	none
KERNELGUARD_CONFIG	Path to TOML config	none
KERNELGUARD_FAIL_OPEN	Allow submissions when CLI unavailable	off
KERNELGUARD_COMMAND	Override CLI command	auto-detect

Test plan

• test_kernelguard.py — 6 tests: mode gating, rejection, fail-open, fail-closed, CLI delegation
• test_backend.py — 2 tests: hacked submission recording with/without pre_sub_id
• test_background_submission_manager.py — 1 test: hacked status propagation
• Manual: enable KERNELGUARD_ENABLED=1 on staging, submit known exploit, verify 400 rejection
• Manual: stop kernelguard binary, verify fail-closed returns 503
• Manual: set KERNELGUARD_FAIL_OPEN=1, verify submissions pass through on outage