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.

controllers/api/api.go

@@ -11,6 +11,8 @@ import (
 	"sort"
 	"time"
 
+	chatclasses "git.golem15.com/golem15/sm-chat-plugin/classes"
+	chanclasses "git.golem15.com/golem15/sm-chatchannels-plugin/classes"
 	"git.golem15.com/golem15/summercms/modules/bouncer"
 	"git.golem15.com/golem15/summercms/modules/phrasebook"
 	"git.golem15.com/golem15/summercms/modules/wire"
@@ -41,6 +43,25 @@ 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
+	// ChatService returns golem15.chat's service, or nil while none is published
+	// (golem15.chat inactive, or no database yet).
+	ChatService func() *chatclasses.Service
+	// Channels returns golem15.chatchannels' channel service, or nil while
+	// none is published.
+	Channels func() *chanclasses.ChannelService
+	// ChatGroup is the code of the user group whose members are in the chat
+	// (jz.bm.chat.group). Empty means nobody is.
+	ChatGroup string
+	// ChatModeratorGroup is the code of the chat's moderator group
+	// (jz.bm.chat.moderator_group).
+	ChatModeratorGroup string
+	// ChatMaxUploadBytes is the total attachment size of one chat message.
+	ChatMaxUploadBytes int
+	// ChatMaxEmbeds is the number of entity embeds one chat message may carry.
+	ChatMaxEmbeds int
+	// ChatBots is whether golem15.chatbots is active, so its is_bot flags
+	// can be read.
+	ChatBots bool
 }
 
 func (h *Handlers) now() time.Time {