Adds the personal producers behind GET /api/notifications without changing its shape:
new assignments (SH-288), unanswered comments on work the user takes part in (SH-289),
@mentions, and decisions on uplifts the user requested (SH-215).
Adds GET /api/notifications, a per-user read model derived from live
work-order state: Unassigned (grouped, High), No Vendor and Aveta Missing
(per work order, Medium) and Vendor Conflict, account-scoped from claims
and ordered by section severity with a fixed reason tie-break.