feat(bridge): add multi-platform bridge — QQ, Telegram, Discord, Webhook

- platform-bridge: unified message model + Adapter interface
- qq/adapter: OBv11 WebSocket protocol with @mention detection
- telegram/discord: stub adapters with send capability
- webhook: HTTP POST receiver for generic webhook integration
- Router: message routing from platform → AI-Core → back
- Group chat rules: 500 char limit, 3s gap, silence keywords
- main.go: conditional adapter loading via env vars

backend/services/platform-bridge/bridge_test.go

@@ -0,0 +1,34 @@
+package bridge
+
+import (
+	"testing"
+	"time"
+)
+
+func TestUnifiedMessage(t *testing.T) {
+	msg := &UnifiedMessage{
+		ID: "123", Platform: "qq", Source: "group-456",
+		SenderID: "789", SenderName: "测试用户",
+		Content: "你好", IsAtBot: true, Timestamp: time.Now(),
+	}
+
+	if msg.Platform != "qq" { t.Error("expected qq") }
+	if !msg.IsAtBot { t.Error("expected is_at_bot") }
+}
+
+func TestGroupChatRules(t *testing.T) {
+	r := DefaultGroupRules()
+	if r.MaxLength != 500 { t.Errorf("MaxLength=%d", r.MaxLength) }
+	if r.MinGap != 3*time.Second { t.Errorf("MinGap=%v", r.MinGap) }
+	if len(r.SilenceKeywords) == 0 { t.Error("expected silence keywords") }
+}
+
+func TestRouter(t *testing.T) {
+	r := NewRouter()
+	var received *UnifiedMessage
+	r.SetMessageHandler(func(msg *UnifiedMessage) { received = msg })
+
+	// Verify handler is set
+	_ = received
+	_ = r
+}