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/router/middlewares_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 
 	"github.com/Rain-kl/Wavelet/internal/model"
+	"github.com/Rain-kl/Wavelet/internal/repository"
 	"github.com/Rain-kl/Wavelet/internal/testhelper"
 	"github.com/gin-gonic/gin"
 )
@@ -21,7 +22,7 @@ func TestCORSMiddleware(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 
 	clearConfigCache := func() {
-		if err := model.InvalidateAllSystemConfigCaches(context.Background()); err != nil {
+		if err := repository.InvalidateAllSystemConfigCaches(context.Background()); err != nil {
 			t.Fatalf("InvalidateAllSystemConfigCaches() error = %v", err)
 		}
 	}