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/cache/logics.go

@@ -0,0 +1,14 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package cache
+
+import (
+	"context"
+
+	"github.com/Rain-kl/Wavelet/internal/repository"
+)
+
+func saveOrUpdateConfig(ctx context.Context, key, value string) error {
+	return repository.SaveOrUpdateSystemConfig(ctx, key, value)
+}