Pause monitors without disabling them

[tomschlick]

Summary

• Add pause and resume from the monitor view and GraphQL, without changing enabled.
• Skip due checks, Check now, post-save checks, queued checks, and heartbeats while a monitor is paused.
• Keep pause out of Terraform config so an apply refreshes the computed status and does not recreate or unpause the monitor.

Test plan

• Pause a monitor from its view page and confirm Resume replaces Pause and Check now.
• Resume it and confirm the status returns to Pending.
• Call pauseMonitor and resumeMonitor.
• Send a heartbeat to a paused monitor and confirm { "ok": true, "paused": true } with no check result.
• Apply a Terraform update to a paused monitor and confirm the stored status stays paused.