[FEATURE] Notification events, mention parsing, and delivery preferences v1

[aernw]

Objective

Add the backend event layer that powers mentions and user notifications.

Scope

• notification event model for comments, reviews, and collaboration actions
• mention parsing or equivalent event generation where relevant
• basic delivery preferences or subscription rules

Out of Scope

• [Features explicitly not included]
• [Future enhancements to defer]

Acceptance Criteria

• notifications are generated from product events rather than manual hacks
• mention-driven collaboration is supported by a reusable backend foundation

Dependencies

Blocked by: (none)

Blocks: (none)

Success Metrics

• [How to measure success - optional]

Roadmap Context

Milestone: M5 - Greenlight Build
Parent workstream(s): #161, #210