email notifications for issue and MR activity #3

Nothing tells an author their issue got a comment or their MR a review — feedback is only visible by polling. SMTP support already exists for verification mail; notify issue/MR authors and participants on comments, reviews, merges, closes. Per-user opt-out later.

Shipped: durable notification queue with webhook-style retries; issue/MR open notifies owner or org admins, comments/reviews/closes/merges notify participants; actor excluded, verified primary addresses only, silent no-op without SMTP. Per-user opt-out remains future work.