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/admin/task/routers_test.go

@@ -20,7 +20,6 @@ import ("bytes"
 	"github.com/Rain-kl/Wavelet/internal/model"
 	"github.com/Rain-kl/Wavelet/internal/task"
 	"github.com/Rain-kl/Wavelet/internal/testhelper"
-	"github.com/Rain-kl/Wavelet/internal/util"
 	"github.com/gin-gonic/gin"
 	"github.com/hibiken/asynq"
 	"github.com/stretchr/testify/assert"
@@ -51,7 +50,7 @@ func setupTestRouter(authUser *model.User) *gin.Engine {
 	// Mock authentication middleware
 	adminGroup.Use(func(c *gin.Context) {
 		if authUser != nil {
-			util.SetToContext(c, oauth.UserObjKey, authUser)
+			oauth.SetToContext(c, oauth.UserObjKey, authUser)
 		}
 		c.Next()
 	})