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/oauth/routers.go

@@ -10,7 +10,7 @@ import ("net/http"
 	"github.com/Rain-kl/Wavelet/pkg/logger"
 	"github.com/gin-contrib/sessions"
 	"github.com/gin-gonic/gin"
-	"github.com/Rain-kl/Wavelet/internal/util"
+
 
 	"github.com/Rain-kl/Wavelet/internal/common/response")
 
@@ -60,7 +60,7 @@ func BuildBasicUserInfo(user *model.User, needChange bool) BasicUserInfo {
 // @Router /api/v1/user-info [get]
 // @Router /api/v1/user/self [get]
 func UserInfo(c *gin.Context) {
-	user, _ := util.GetFromContext[*model.User](c, UserObjKey)
+	user, _ := GetFromContext[*model.User](c, UserObjKey)
 	session := sessions.Default(c)
 	needChange := session.Get("need_change_password") == true