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/apps/risk_control/middleware.go

@@ -14,7 +14,6 @@ import (
 	"github.com/Rain-kl/Wavelet/internal/config"
 	"github.com/Rain-kl/Wavelet/internal/db/idgen"
 	"github.com/Rain-kl/Wavelet/internal/model"
-	"github.com/Rain-kl/Wavelet/internal/util"
 	"github.com/gin-gonic/gin"
 )
 
@@ -39,7 +38,7 @@ func RiskControlMiddleware() gin.HandlerFunc {
 		c.Next()
 
 		// 3. 后置身份检查：仅记录通过认证的请求
-		userObj, exists := util.GetFromContext[*model.User](c, oauth.UserObjKey)
+		userObj, exists := oauth.GetFromContext[*model.User](c, oauth.UserObjKey)
 		if !exists || userObj == nil {
 			return
 		}