feat: add Cronitor cron job and uptime monitoring provider

[selenaalpha77-sketch]

Summary

Adds a new provider for Cronitor, a monitoring platform for cron jobs, heartbeats, scheduled tasks, and uptime checks. Cronitor is widely used by engineering teams to ensure background jobs and scheduled tasks run on time.

• Pull failing monitors from the Cronitor REST API (HTTP Basic Auth with API key)
• Receive real-time webhook notifications when jobs fail, miss their schedule, or recover
• Support all Cronitor monitor types: job, heartbeat, and uptime monitors
• Map Cronitor monitor states (failing, degraded, healthy, paused) to Keep severities and statuses
• Map Cronitor webhook event types (monitor.failure, monitor.recovery, heartbeat.missed) to Keep alert statuses
• Filter monitors by environment via optional environment config parameter
• Full alerts_mock.py for webhook payload testing
• MDX documentation with setup for both webhook and pull modes

Provider files

• keep/providers/cronitor_provider/cronitor_provider.py
• keep/providers/cronitor_provider/__init__.py
• keep/providers/cronitor_provider/alerts_mock.py
• docs/providers/documentation/cronitor-provider.mdx

Test plan

• Configure provider with a valid Cronitor API key and verify validate_scopes returns read_monitors: True
• Verify _get_alerts() returns failing monitors with correct severity/status mapping
• Verify environment filter works when environment config is set
• Send a test webhook payload matching alerts_mock.py and verify _format_alert produces a correct AlertDto
• Verify pagination works correctly for accounts with more than 100 monitors

🤖 Generated with Claude Code

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