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.

config/config.yaml

@@ -9,3 +9,18 @@ dashboard_limit: 5
 # the calendar feed URL and the course links inside the feed. Empty means
 # the scheme and Host of the request.
 public_url: ""
+
+# The student chat (golem15.chat, golem15.chatchannels). Membership is the
+# user group whose code is chat.group; nobody is added to it automatically,
+# members are assigned in the backend. Members of chat.moderator_group may
+# delete other people's messages in channels. Env keys:
+# SUMMER_JZ__BM__CHAT__GROUP, SUMMER_JZ__BM__CHAT__MODERATOR_GROUP,
+# SUMMER_JZ__BM__CHAT__MAX_UPLOAD_BYTES, SUMMER_JZ__BM__CHAT__MAX_EMBEDS.
+chat:
+  group: studenci
+  moderator_group: moderatorzy
+  # Total size of one message's attachments (60 MiB). It stays under nginx's
+  # client_max_body_size 64m so the multipart envelope still fits.
+  max_upload_bytes: 62914560
+  # Entity embeds (deadline, class, reading) per message.
+  max_embeds: 3