Stage 7: Checker triggers — Emails on automatic transitions

[bakhterets]

Integrate notification enqueuing into the checker layer to send emails on automatic status transitions.

Files: internal/checker/maintenance.go, internal/checker/checker.go

Tasks:

• Enqueue kind status_changed, actor checker.
• Publish inside the winning optimistic-lock transaction only.
• On version conflict, publish nothing.
• One notification per real transition (no duplicates from backfill).
• Verify: integration test for transition and conflict paths.

Deliverable: Checker triggers send notifications on automatic maintenance transitions.

Depends on: Stage 4: Notification core, Stage 5: Transaction-safe writes