Notifications specification

Users can opt into email notifications when a task is assigned or becomes overdue. Preferences are stored per user and default to disabled. Assignment events already exist; overdue detection runs every fifteen minutes.

Requirements:

• Add preference read/update endpoints with boundary validation.
• Publish notification jobs from assignment and overdue flows.
• Deduplicate jobs by user, task, event type, and event version.
• Send email through the existing provider adapter.
• Record delivery status without storing message bodies.
• Feature flag the sending path; disabled remains the safe default.

Verification must include preference API tests, job deduplication tests, provider-adapter integration tests, and one end-to-end assignment scenario. No SMS, push notifications, or notification history UI is in scope.