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/api.go

@@ -43,6 +43,10 @@ type Handlers struct {
 	// (jz.bm.public_url), used for the calendar feed URL and the links in
 	// it. Empty means the scheme and Host of the request.
 	PublicURL string
+	// AppURL is the application URL (app.url). The MCP link rewriter
+	// (jz.bm.mcp-links) prefers it to PublicURL when it makes the links of
+	// an MCP answer absolute.
+	AppURL string
 	// ChatService returns golem15.chat's service, or nil while none is published
 	// (golem15.chat inactive, or no database yet).
 	ChatService func() *chatclasses.Service