- BindConversationAccountRequest: restrict bindingType to 1-4
- ConversationService.bindAccount: upsert binding by account+conversation+type with sortOrder and status=1
- SendService.selectSendAccount: prefer send-primary(1), fallback send-backup(2), status=1, tenant/channel match, sortOrder asc
- SendService.resolveConversationTarget/resolvePersonTarget: verify account tenant, channel type and status; ensure person identity channel type matches for single chat