feat: MCP view of BM answers (absolute links, no material body by default)

- jz.bm.mcp-links on the token twins and the management API: under MCP,
  url and *_url strings starting with a single / get the app base
- material answers under MCP leave the body out unless include_body,
  marking it with body_omitted; write answers never echo it
- jz.bm.manage keys a dispatched call by user
- bm_materials and bm_material student tools
Plain HTTP answers are unchanged.

controllers/api/manage_files.go

@@ -112,7 +112,7 @@ func (h *Handlers) AttachMaterialFiles(w http.ResponseWriter, r *http.Request) {
 		}
 		return
 	}
-	h.writeMaterialDetail(w, db, m.ID, http.StatusCreated)
+	h.writeMaterialDetail(w, r, db, m.ID, http.StatusCreated)
 }
 
 // storeParts stores and binds every "file" part in one transaction. A