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/chat.go

@@ -0,0 +1,36 @@
+package api
+
+import (
+	"net/http"
+
+	userclasses "git.golem15.com/golem15/sm-user-plugin/classes"
+)
+
+// chatBootstrapJSON is the answer of /chat. A user outside the chat group
+// gets only member: false.
+type chatBootstrapJSON struct {
+	Member bool `json:"member"`
+}
+
+// Chat answers GET /chat, the chat bootstrap: whether the caller is in the
+// chat group.
+func (h *Handlers) Chat(w http.ResponseWriter, r *http.Request) {
+	userID, ok := h.caller(w, r)
+	if !ok {
+		return
+	}
+	db, ok := h.db(w, r)
+	if !ok {
+		return
+	}
+	member := false
+	if h.ChatGroup != "" {
+		var err error
+		member, err = userclasses.HasGroupCode(r.Context(), db, userID, h.ChatGroup)
+		if err != nil {
+			writeOpaque500(w)
+			return
+		}
+	}
+	writeData(w, chatBootstrapJSON{Member: member})
+}