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/model/templates.go

@@ -5,14 +5,10 @@ package model
 
 import (
 	"bytes"
-	"context"
 	"errors"
-	"fmt"
 	"strings"
 	"text/template"
 	"time"
-
-	"github.com/Rain-kl/Wavelet/internal/db"
 )
 
 // Template 邮件/消息模板实体
@@ -90,17 +86,3 @@ func (t *Template) Render(data any) (string, string, error) {
 
 	return subject, bodyBuf.String(), nil
 }
-
-// RenderTemplate 渲染指定模板。模板不存在或渲染失败时返回错误，由调用方决定如何处理。
-func RenderTemplate(ctx context.Context, key string, data any) (string, string, error) {
-	var t Template
-	if err := db.DB(ctx).Where("key = ?", key).First(&t).Error; err != nil {
-		return "", "", fmt.Errorf(errTemplateUnavailable, key, err)
-	}
-
-	subject, body, err := t.Render(data)
-	if err != nil {
-		return "", "", fmt.Errorf(errTemplateRenderFailed, key, err)
-	}
-	return subject, body, nil
-}