fix: revoke chat access as soon as a user leaves the chat group

The BM chat context implements golem15.chat's LiveMembership, so every
participant check also requires chat-group membership: a removed user
loses # ogólny, their DMs, uploads, subscriptions and chat:message
delivery at once instead of after the next participant sync.

classes/chat_context.go

@@ -12,6 +12,7 @@ import (
 var (
 	_ chatcontracts.Context          = (*ChatContext)(nil)
 	_ chatcontracts.MessageModerator = (*ChatContext)(nil)
+	_ chatcontracts.LiveMembership   = (*ChatContext)(nil)
 	_ chancontracts.AccessResolver   = (*ChatAccess)(nil)
 )
 
@@ -134,6 +135,12 @@ func (c *ChatContext) CanModerateMessage(ctx context.Context, userID uint, conve
 	return IsChatModerator(ctx, c.db, userID, c.moderatorGroup)
 }
 
+// RequireLiveMembership is always true: golem15.chat checks HasMember on
+// every participant check of the BM chat, so someone taken out of the
+// chat group loses # ogólny and their DMs at once, not when their
+// participant rows are pruned.
+func (c *ChatContext) RequireLiveMembership() bool { return true }
+
 // HasMember reports whether userID is in the chat group.
 func (c *ChatContext) HasMember(ctx context.Context, userID uint) (bool, error) {
 	if userID == 0 || c.group == "" {

classes/chat_context_test.go

@@ -92,6 +92,9 @@ func TestChatContext(t *testing.T) {
 	if _, err := c.HasMember(ctx, 13); err == nil {
 		t.Error("HasMember with a failing check returned no error")
 	}
+	if !c.RequireLiveMembership() {
+		t.Error("RequireLiveMembership = false, want true: removal from the chat group must revoke access at once")
+	}
 	if !c.CanActFor(ctx, 7, 7) || c.CanActFor(ctx, 7, 8) {
 		t.Error("CanActFor is not self-only")
 	}