Stage 6: API triggers — Emails on create/patch

[bakhterets]

Integrate notification enqueuing into the API layer to send emails on maintenance create/patch operations.

Files: internal/api/v2/v2.go, internal/api/api.go, internal/api/routes.go

Tasks:

• Wrap each maintenance write + enqueue in one transaction.
• Generate a fresh change_id per successful change.
• Enqueue only for maintenance, only on success, only when enabled.
• Map kind by resulting status: pending_review, reviewed, status_changed.
• Verify: no outbox row on 400/403/409/500; integration test for recipients.

Deliverable: API endpoints trigger notifications on successful maintenance changes.

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