feat: 同平台多账号支持 — adapter_name 贯穿全链路

- UnifiedMessage 新增 AdapterName 字段
- router 从适配器 ConfigName() 捕获 adapter 实例名
- forwardToAICore source 新增 adapter_name
- ai-core SetBotUID key 优先用 adapter_name，回退 platform
- 更新优化大纲加入 P4.1

Co-Authored-By: Claude <noreply@anthropic.com>

backend/platform-bridge/internal/bridge/unified.go

@@ -29,6 +29,7 @@ type UnifiedMessage struct {
 	GroupName           string      `json:"group_name,omitempty"`            // resolved group name for group chats
 	OriginalRawMessage  interface{} `json:"-"`                               // preserved for SendMessage wiring
 	BotUID              string      `json:"-"`                               // bot's own platform UID, set by router
+	AdapterName         string      `json:"-"`                               // adapter config name (e.g. "qq-main"), set by router
 }
 
 // Attachment represents a file/image/voice/video attachment.