refactor: 重构项目结构

- 将server端相关依赖单独防止在server中

interval/server/model/doc.go

@@ -0,0 +1,2 @@
+// Package model defines core domain models of the push service.
+package model

interval/server/model/message.go

@@ -0,0 +1,7 @@
+package model
+
+type Message struct {
+	Topic     Topic
+	Content   []byte
+	Timestamp int64
+}

interval/server/model/subscription.go

@@ -0,0 +1,6 @@
+package model
+
+type Subscription struct {
+	Topic    Topic
+	ClientID string
+}

interval/server/model/topic.go

@@ -0,0 +1,7 @@
+package model
+
+type Topic string
+
+func (t Topic) Valid() bool {
+	return t != ""
+}