feat: search materials through beachcomber

- Material is a beachcomber Searchable (index jz_bm_materials) in the
  hand-written models/material_search.go; jz.bm requires summercms v0.6.4
- classes.MaterialIndexer asks Typesense for up to 250 candidates with
  body highlights; the app-wide Gate is on only for jz.bm-marked contexts
- GET /_bm/api/v1/materials takes q: candidates are re-gated, counted and
  paged in SQL in rank order, each row with at most one plain snippet
- search failure answers 503 Search is unavailable.; without q the list
  is unchanged

controllers/api/export_test.go

@@ -2,3 +2,11 @@ package api
 
 // DecodeInline exposes decodeInline to the api_test package.
 var DecodeInline = decodeInline
+
+// PickSnippet, FlattenSnippet and RankOrder expose the search helpers to the
+// api_test package.
+var (
+	PickSnippet    = pickSnippet
+	FlattenSnippet = flattenSnippet
+	RankOrder      = rankOrder
+)