refactor(api): extract repository layer and thin HTTP handlers

Introduce internal/repository for data access and cache-backed system
config reads. Move business logic into logics.go across admin push,
user, template, cache, system_config, and upload/handler packages.

Remove Gin from internal/util by relocating request-scoped helpers to
oauth/gin_context.go. Propagate request context for config lookups in
user flows. Slim model entities and delete model-level DB/cache helpers.

Wire handlers to logics/repository so targeted packages no longer call
db.DB directly. Update admin router tests to use ErrorHandlerMiddleware.

internal/util/context.go

@@ -1,24 +0,0 @@
-// Copyright 2025 linux.do
-// Copyright 2026 Arctel.net
-// SPDX-License-Identifier: Apache-2.0
-
-// Package util 提供通用工具函数
-package util
-
-import "github.com/gin-gonic/gin"
-
-// GetFromContext 从上下文获取指定类型的值
-func GetFromContext[T any](c *gin.Context, key string) (T, bool) {
-	value, exists := c.Get(key)
-	if !exists {
-		var zero T
-		return zero, false
-	}
-	typed, ok := value.(T)
-	return typed, ok
-}
-
-// SetToContext 设置值到上下文
-func SetToContext[T any](c *gin.Context, key string, value T) {
-	c.Set(key, value)
-}

internal/util/custom_types.go

@@ -2,6 +2,7 @@
 // Copyright 2026 Arctel.net
 // SPDX-License-Identifier: Apache-2.0
 
+// Package util provides framework-agnostic helper types and HTTP utilities.
 package util
 
 import (