feat: host the BM chat context

When golem15.chat is active, Boot registers the context alias bm (id 1)
on its registry. Membership is the user group jz.bm.chat.group (default
studenci); CanOverseeChats is always false, so nobody reads other
people's DMs or edits their messages; the broadcast channel is
chat:context:1, never empty.

GET /_bm/api/v1/chat answers the caller's membership behind jwt.auth and
the per-user bucket jz.bm.chat (240 a minute). The chat config block
(group, moderator_group, max_upload_bytes, max_embeds) is added. jz.bm
still Requires only golem15.user; all chat wiring is gated by HasPlugin.

plugin.go

@@ -69,14 +69,18 @@ func (p *Plugin) Register(app *backpack.App) error {
 
 // Boot keeps the application, so route handlers and commands can reach its
 // services, such as the database, when they run, and wires the realtime
-// service. The management API uses the shared user.api_token middleware
-// owned by golem15.user.
+// service and, when golem15.chat is active, the BM chat context. The
+// management API uses the shared user.api_token middleware owned by
+// golem15.user.
 func (p *Plugin) Boot(app *backpack.App) error {
 	p.app = app
 	if app != nil {
 		if err := p.wireRealtime(app); err != nil {
 			return err
 		}
+		if err := p.registerChat(app); err != nil {
+			return err
+		}
 	}
 	return nil
 }