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/json.go

@@ -213,6 +213,10 @@ type materialListJSON struct {
 	Tags        []tagJSON  `json:"tags"`
 	FilesCount  int64      `json:"files_count"`
 	IsPrivate   bool       `json:"is_private"`
+	// Snippet is the matched passage of a search result (*snippetJSON, or
+	// a plain string through MCP). It is nil, and the key absent, without
+	// q or without a highlight.
+	Snippet any `json:"snippet,omitempty"`
 }
 
 func newMaterialListJSON(m models.Material, files int64) materialListJSON {