chore(init): 初始化解忧商行项目并完成 Phase 0 基座

backend/internal/modules/supportgroup/errors.go

@@ -0,0 +1,9 @@
+package supportgroup
+
+import "errors"
+
+var (
+	ErrDependencyUnavailable = errors.New("dependency unavailable")
+	ErrInvalidGroup          = errors.New("invalid support group")
+	ErrProtectedGroup        = errors.New("protected support group")
+)