fix: clean up worker claims during shutdown

[MuncleUscles]

Summary

• remove the worker service's custom SIGTERM/SIGINT handler so Uvicorn can run ASGI lifespan shutdown
• on shutdown, stop claiming work, terminate validators/GenVM subprocesses quickly, cancel active tasks, then release/reset this worker's owned transaction claims
• preserve finalization-ready transaction state while resetting in-progress consensus statuses back to retryable PENDING, with recovery-count exhaustion still canceling repeated failures
• add DB coverage for owned claim reset, finalization-state preservation, and recovery exhaustion during shutdown cleanup

Context

Rally prod canary evidence showed the worker received /stop and SIGTERM, the worker supervisor stopped quickly, but the process stayed alive until Kubernetes killed it with exit 137. This PR makes termination deterministic and prevents a terminating worker from stranding owned transaction claims when GenVM or active work does not complete inside the pod grace period.

Verification

• python3 -m py_compile backend/consensus/worker.py backend/consensus/worker_service.py tests/db-sqlalchemy/test_worker_shutdown_claim_cleanup.py
• pytest tests/db-sqlalchemy/test_worker_shutdown_claim_cleanup.py could not run locally because this environment is missing alembic (ModuleNotFoundError: No module named 'alembic').
• commit hooks passed: trailing whitespace, large files, JSON/YAML checks, merge conflict check, branch check, autoflake, black, eslint/prettier skips for no frontend files

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[github-actions]

This PR targeted main, which is only the default/static branch.

I retargeted it to v0.123-dev, the active development branch. Pushes to v0.123-dev automatically fast-forward main.

[sonarqubecloud]

Quality Gate failed

Failed conditions
11 Security Hotspots
79.1% Coverage on New Code (required ≥ 80%)
C Security Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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