Attach a channel to an alarm

Attach another notification channel to an alarm, so one condition reaches several destinations (page Slack and email the on-call, say).

Accepts the same channel shape as alarm creation: { "id": <channel id> } to reuse an existing channel, or inline type + config to create one. Inline channels identical to an existing row are deduped rather than duplicated.

Idempotent — re-attaching a channel that is already linked returns already_linked: true and changes nothing.

Gated by the alarm, not the channel: alarms.manage for an org-level alarm, projects.alarms.manage on the owner for a project alarm. Channels themselves are org-wide, so a project-scoped caller can point their alarm at any existing channel — attaching one is a routing decision about their alarm, whereas creating or deleting the channel row needs notification_channels.manage.