fix: 消息日志增强 + 历史消息抑制 + SSE实时追踪 + 群聊上下文优化

- 日志：收/发消息均显示群名称，管理员显示真实QQ昵称而非"开拓者"
- 历史消息：服务重启后NapCat回放的历史消息不再触发回复，静默注入上下文
- 消息时间戳：转发给AI时附带【消息时间: HH:MM:SS (XmXs前)】标记
- ♪ 分割符：QQ消息支持♪作为句子断点
- AI-Core SSE端点：全链路追踪实时推送，ethend不再5秒轮询
- 群聊上下文：AI-Core明确被告知消息来自群聊，以实际发送者为主语

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -23,10 +23,12 @@ type UnifiedMessage struct {
 	Timestamp time.Time   `json:"timestamp"`
 
 	// Routing metadata.
-	RouteType         string      `json:"route_type,omitempty"`          // "normal", "silent", "admin_mention"
-	OriginalSenderUID string      `json:"original_sender_uid,omitempty"` // preserved before identity mapping
-	OriginalRawMessage interface{} `json:"-"`                             // preserved for SendMessage wiring
-	BotUID            string      `json:"-"`                             // bot's own platform UID, set by router
+	RouteType           string      `json:"route_type,omitempty"`            // "normal", "silent", "admin_mention"
+	OriginalSenderUID   string      `json:"original_sender_uid,omitempty"`   // preserved before identity mapping
+	OriginalSenderName  string      `json:"original_sender_name,omitempty"`  // preserved before identity mapping
+	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
 }
 
 // Attachment represents a file/image/voice attachment.