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/params.go

@@ -37,6 +37,7 @@ var (
 		{Field: "type", Rules: append(lagoon.ParseRules("nullable"), lagoon.In(models.MaterialTypes...))},
 		{Field: "page", Rules: lagoon.ParseRules("nullable|integer|min:1")},
 		{Field: "per_page", Rules: lagoon.ParseRules("nullable|integer|between:1,100")},
+		{Field: "q", Rules: lagoon.ParseRules("nullable|string|max:100")},
 	}
 )