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/ai-core/internal/background/thinker_test.go

@@ -42,8 +42,8 @@ func TestExtractProactiveMessage_OBv11Group(t *testing.T) {
 	if target == nil {
 		t.Fatal("expected target, got nil")
 	}
-	if target.Platform != "qq" {
-		t.Errorf("expected platform 'qq', got %q", target.Platform)
+	if target.Platform != "obv11" {
+		t.Errorf("expected platform 'obv11', got %q", target.Platform)
 	}
 	if target.ChatType != "group" {
 		t.Errorf("expected chatType 'group', got %q", target.ChatType)