Fold workflow cover into list/search payload to avoid per-card N+1 fetches

[Ma77Ball]

Task Summary

Each workflow card fetches its cover via a separate GET /{wid}/cover (a DB hit plus an access check), so a dashboard or hub page of N cards issues N requests, which scales poorly on a busy dashboard. Fold a hasCover flag (or the cover itself) into the existing workflow-list/search payload so cards render directly or gate a lazy fetch, instead of fetching unconditionally on init. Follow-up from PR #5704; the per-card stale-cover flash and out-of-order resolution were already fixed there via switchMap plus eager reset. Relevant code: WorkflowCoverService.getCover and card-item.component.ts (frontend), WorkflowResource.getCoverImage (backend).

Task Type

• Performance

[Ma77Ball]

Folding this into PR #5704 instead of tracking separately.

[Ma77Ball]

Reopening: keeping the N+1 fetch-volume work as a separate follow-on PR rather than folding it into #5704.