[codex] add cron diagnostics and scheduler failure state

[zachyzissou]

Summary

• persist generation failure metadata to state (last_error, last_failure_at, last_failure_context, consecutive_failures)
• clear failure metadata on successful generation
• expose cron diagnostics in /status (cron_supported, cron_error) and include failure/freshness details
• expose cron diagnostics in /health (checks.cron_schedule plus cron payload)
• make scheduler fail fast on invalid/unsupported cron config (no silent fallback); recheck periodically and continue stale watchdog behavior
• add integration tests for cron/failure diagnostics and failure-state persistence

Validation

• python3 -m black app/server.py test_integration.py
• python3 -m py_compile app/server.py test_integration.py
• npm run -s lint:js
• npm run -s lint:md

Notes

• local runtime tests were not executed here because this environment has Python 3.9 while the project targets 3.11+.

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