refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/upload/cache/access_cache_test.go

@@ -0,0 +1,89 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package cache
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/testhelper"
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/shared"
+	uploadstorage "github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/storage"
+)
+
+func TestLoadMigrationAccessStateCachesResult(t *testing.T) {
+	_, _, cleanup := testhelper.SetupTestEnvironment(t)
+	defer cleanup()
+	ResetAccessCaches()
+
+	ctx := context.Background()
+	first := uploadstorage.LoadMigrationAccessState(ctx)
+	second := uploadstorage.LoadMigrationAccessState(ctx)
+
+	if first.ReadOnly != second.ReadOnly {
+		t.Fatalf("readOnly mismatch: first=%v second=%v", first.ReadOnly, second.ReadOnly)
+	}
+	if first.HasTarget != second.HasTarget {
+		t.Fatalf("hasTarget mismatch: first=%v second=%v", first.HasTarget, second.HasTarget)
+	}
+}
+
+func TestIsFilePublicUsesCachedWhitelist(t *testing.T) {
+	_, _, cleanup := testhelper.SetupTestEnvironment(t)
+	defer cleanup()
+	ResetAccessCaches()
+
+	ctx := context.Background()
+	if !IsFilePublic(ctx, "avatar") {
+		t.Fatal("expected avatar to be public by default")
+	}
+	if IsFilePublic(ctx, "attachment") {
+		t.Fatal("expected attachment to be private by default")
+	}
+	if !IsFilePublic(ctx, "AVATAR") {
+		t.Fatal("expected whitelist lookup to be case-insensitive")
+	}
+}
+
+func TestResetAccessCachesRefreshesWhitelist(t *testing.T) {
+	dbConn, _, cleanup := testhelper.SetupTestEnvironment(t)
+	defer cleanup()
+	ResetAccessCaches()
+
+	ctx := context.Background()
+	if !IsFilePublic(ctx, "avatar") {
+		t.Fatal("expected seeded avatar whitelist before reset")
+	}
+
+	if err := dbConn.Table("w_system_configs").Where("key = ?", "file_access_whitelist").Update("value", `["attachment"]`).Error; err != nil {
+		t.Fatalf("update whitelist config: %v", err)
+	}
+
+	ResetAccessCaches()
+	if !IsFilePublic(ctx, "attachment") {
+		t.Fatal("expected attachment to be public after whitelist refresh")
+	}
+	if IsFilePublic(ctx, "avatar") {
+		t.Fatal("expected avatar to be private after whitelist refresh")
+	}
+}
+
+func TestAccessCacheTTLExpires(t *testing.T) {
+	_, _, cleanup := testhelper.SetupTestEnvironment(t)
+	defer cleanup()
+	ResetAccessCaches()
+
+	ctx := context.Background()
+	_ = loadFileAccessWhitelist(ctx)
+
+	fileAccessWhitelistMu.Lock()
+	fileAccessWhitelistCheckedAt = time.Now().Add(-time.Duration(shared.AccessCacheTTL)*time.Second - time.Second)
+	fileAccessWhitelistMu.Unlock()
+
+	// Should still work after TTL by reloading from config.
+	if !IsFilePublic(ctx, "avatar") {
+		t.Fatal("expected whitelist reload after TTL expiration")
+	}
+}