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/template/logics.go

@@ -0,0 +1,78 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package template
+
+import (
+	"context"
+	"errors"
+
+	"github.com/Rain-kl/Wavelet/internal/model"
+	"github.com/Rain-kl/Wavelet/internal/repository"
+)
+
+func createTemplate(ctx context.Context, req CreateTemplateRequest) (model.Template, error) {
+	exists, err := repository.TemplateExistsByKey(ctx, req.Key)
+	if err != nil {
+		return model.Template{}, err
+	}
+	if exists {
+		return model.Template{}, errors.New(TemplateKeyExists)
+	}
+
+	tmpl := model.Template{
+		Key:         req.Key,
+		Name:        req.Name,
+		Type:        req.Type,
+		Subject:     req.Subject,
+		Content:     req.Content,
+		Description: req.Description,
+		IsSystem:    false,
+	}
+	if err := tmpl.Validate(); err != nil {
+		return model.Template{}, err
+	}
+	if err := repository.CreateTemplate(ctx, &tmpl); err != nil {
+		return model.Template{}, err
+	}
+	return tmpl, nil
+}
+
+func listTemplates(ctx context.Context) ([]model.Template, error) {
+	return repository.ListTemplates(ctx)
+}
+
+func getTemplate(ctx context.Context, key string) (model.Template, error) {
+	return repository.GetTemplateByKey(ctx, key)
+}
+
+func updateTemplate(ctx context.Context, key string, req UpdateTemplateRequest) (model.Template, error) {
+	tmpl, err := repository.GetTemplateByKey(ctx, key)
+	if err != nil {
+		return model.Template{}, err
+	}
+
+	tmpl.Name = req.Name
+	tmpl.Type = req.Type
+	tmpl.Subject = req.Subject
+	tmpl.Content = req.Content
+	tmpl.Description = req.Description
+	if err := tmpl.Validate(); err != nil {
+		return model.Template{}, err
+	}
+	if err := repository.SaveTemplate(ctx, &tmpl); err != nil {
+		return model.Template{}, err
+	}
+	return tmpl, nil
+}
+
+func deleteTemplate(ctx context.Context, key string) error {
+	tmpl, err := repository.GetTemplateByKey(ctx, key)
+	if err != nil {
+		return err
+	}
+	if tmpl.IsSystem {
+		return errors.New(SystemTemplateCannotDelete)
+	}
+	return repository.DeleteTemplate(ctx, &tmpl)
+}