Explain blocked schedule runs and let a failed document be processed again

Schedule history now shows a readable reason instead of the engine's raw code,
and under the newest run stopped by knowledge_not_ready it lists the Library
material that blocks it, with a link to fix it in Learning Studio. The list
refreshes while that material is still processing or building.

Upload gets a "Process again" button for a failed document, using the engine's
new reprocess endpoint; a knowledge base the document already has is kept. A
document being processed again no longer shows as "In the Brain".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/api/engine.ts

@@ -79,6 +79,8 @@ export interface KnowledgeDetail {
 export const knowledgeDetail = (id: number) => request<KnowledgeDetail>(`/upload/${id}/knowledge`)
 
 // Agree to the quoted price, build after a top-up, or retry a failed build. Spends workspace credits (402 when short).
+// Reads a failed document's stored file again, for free. A knowledge base it already has is kept.
+export const reprocessDocument = (id: number) => request<{ id: number; status: string }>(`/upload/${id}/reprocess`, { method: 'POST' })
 export const buildKnowledge = (id: number) => request<{ status: string }>(`/upload/${id}/knowledge/build`, { method: 'POST' })
 
 // The engine asks whether an upload is the next version of a similar document before building it.