feat: index the text of attached PDFs

- jz_bm_material_file_texts caches the pdftotext output per file
- the River job jz.bm.extract_material_text extracts and resyncs
- the management multipart attach queues it inside its transaction
- file_texts in the document and a file snippet with its name

controllers/api/export_test.go

@@ -9,4 +9,5 @@ var (
 	PickSnippet    = pickSnippet
 	FlattenSnippet = flattenSnippet
 	RankOrder      = rankOrder
+	HasFileSnippet = hasFileSnippet
 )