refactor: QQ → OBv11 重命名 + 平台格式统一抽象

- 所有对外称呼从 QQ 改为 OBv11（注释/提示词/日志/配置项）
- 新增 PlatformFormat 结构体，统一管理平台消息标记格式
- defaultPlatformFormats() 注册表替代硬编码 qqTargetRe
- extractProactiveMessage 改为 Thinker 方法，遍历格式注册表匹配
- 配置项重命名: QQ_BOT_PORT → OBV11_BOT_PORT, QQBotPort → OBv11BotPort
- 标记格式: 【QQ群聊】→【OBv11群聊】、【QQ私聊】→【OBv11私聊】

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

backend/ai-core/cmd/main.go

@@ -932,7 +932,7 @@ func handleChat(
 	}
 
 	// Admin private messages: redirect to the main admin session so conversation
-	// history is shared across platforms (QQ, web UI, etc.).
+	// history is shared across platforms (OBv11, web UI, etc.).
 	if req.UserID == "admin" && req.Source.ChannelType == "direct" && adminSessionID != "" {
 		req.SessionID = adminSessionID
 	}