refactor: 彻底消除硬编码 qq — 平台标识统一为 obv11

- PlatformName() 返回 "obv11"
- platformFormats key: "obv11"
- 所有 platform == "qq" 检查 → "obv11"
- session prefix: platform_qq_ → platform_obv11_
- env var PLATFORM_CHANNELS: obv11:group:xxx
- ethend 平台标识同步更新
- 注释和测试用例同步

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

backend/platform-bridge/internal/adapter/qq/channels.go

@@ -65,7 +65,7 @@ func (a *Adapter) FetchGroups() []bridge.ChannelInfo {
 			a.SetGroupName(g.GroupID, g.GroupName)
 		}
 		channels = append(channels, bridge.ChannelInfo{
-			Platform:    "qq",
+			Platform:    "obv11",
 			ChannelType: "group",
 			ChannelID:   fmt.Sprintf("%d", g.GroupID),
 			ChannelName: g.GroupName,
@@ -124,7 +124,7 @@ func (a *Adapter) FetchFriends() []bridge.ChannelInfo {
 			name = f.Nickname
 		}
 		channels = append(channels, bridge.ChannelInfo{
-			Platform:    "qq",
+			Platform:    "obv11",
 			ChannelType: "private",
 			ChannelID:   fmt.Sprintf("%d", f.UserID),
 			ChannelName: name,