[3c] Ingestion status UI

[andrmaz]

What to build

Admin dashboard shows per-source ingestion status (pending / running / completed / failed) derived from the BullMQ job state and Document/Chunk counts. Failed states surface actionable error details so admins can diagnose and retry.

Acceptance criteria

• Admin dashboard lists sources with their current ingestion status.
• Status reflects real queue job state (pending / running / completed / failed).
• Failed sources display an error reason, not just a generic failure state.
• Chunk count per source is visible on completion.
• UI updates without requiring a manual page refresh (polling or SSE acceptable for MVP).

Blocked by

• [3b] Async ingestion worker — chunk, embed, index #10 [3b] Async ingestion worker — chunk, embed, index