Dashboard: engagement stats (GET /api/user/engagement) #65

Part of #62 — Dashboard & data-management surface

"Aggregate dig/push engagement received on the current user's own messages."

Verify bearer access first — this endpoint previously returned Unauthorized to a valid
bearer token, suggesting it is cookie-gated. If it is genuinely web-only, close this issue as
out-of-scope and record that on the epic.

If it is reachable: show the aggregate on the dashboard, and consider per-message counts on the
own-messages list.

Tests: parse the aggregate payload; empty state.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.