doc: 为现有模块补足模块文档

interval/api/doc.go

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+// Package api defines the HTTP control plane of the push service
+package api

interval/api/handler/doc.go

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+// Package handler contains HTTP request handlers for the REST API.
+package handler

interval/model/doc.go

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+// Package model defines core domain models of the push service.
+package model

interval/server/doc.go

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+// Package server provides HTTP server abstractions.
+package server