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/repository/upload_stat.go

@@ -0,0 +1,20 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package repository
+
+import (
+	"context"
+
+	"github.com/Rain-kl/Wavelet/internal/db"
+	"github.com/Rain-kl/Wavelet/internal/model"
+)
+
+// ListUploadStats returns all upload statistics rows.
+func ListUploadStats(ctx context.Context) ([]model.UploadStat, error) {
+	var stats []model.UploadStat
+	if err := db.DB(ctx).Find(&stats).Error; err != nil {
+		return nil, err
+	}
+	return stats, nil
+}