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/handlers_test.go

@@ -152,6 +152,7 @@ func TestQueryValidation(t *testing.T) {
 		{"materials course negative", h.Materials, "/?course=-1", "course"},
 		{"materials meeting zero", h.Materials, "/?meeting=0", "meeting"},
 		{"materials tag too long", h.Materials, "/?tag=" + longTag(), "tag"},
+		{"materials q too long", h.Materials, "/?q=" + longTag(), "q"},
 	}
 	for _, tc := range cases {
 		code, body := serve(t, tc.fn, http.MethodGet, tc.target, user, nil)