Escalation was calling SendAlert() which always sends to all enabled
channels, ignoring the per-level channel selection (email/webhook/all).
Add SendAlertToChannels() that snapshots only the requested channel
configs and uses a distinct "_escalation" queue type so the dequeue
handler skips cooldown writes — preventing interference with the alert
manager's own re-notify cadence.
Tests for annotateResolvedMetadata, resolveAppriseNotificationType,
normalizeQueueType, and resolvedTimeFromAlerts (44 test cases).
Coverage 38.0% → 39.0%.