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/diskcache/cache_test.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/Rain-kl/Wavelet/internal/db"
 	"github.com/Rain-kl/Wavelet/internal/model"
+	"github.com/Rain-kl/Wavelet/internal/repository"
 	"github.com/Rain-kl/Wavelet/internal/testhelper"
 )
 
@@ -31,7 +32,7 @@ func TestDiskCacheReloadConfig(t *testing.T) {
 
 	// Invalidate Redis config cache to force DB reload
 	if db.Redis != nil {
-		db.Redis.Del(context.Background(), db.PrefixedKey(model.SystemConfigRedisHashKey))
+		db.Redis.Del(context.Background(), db.PrefixedKey(repository.SystemConfigRedisHashKey))
 	}
 
 	// Reload config