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/infra/storage/diskcache/cache_test.go

@@ -0,0 +1,49 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package diskcache
+
+import (
+	"context"
+	"os"
+	"testing"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/testhelper"
+	cache "github.com/Rain-kl/Wavelet/backend/plugins/infra/cache"
+)
+
+func TestDiskCacheReloadConfig(t *testing.T) {
+	dbConn, _, cleanup := testhelper.SetupTestEnvironment(t)
+	defer cleanup()
+
+	testDir := "uploads/test_diskcache_reload"
+	defer func() { _ = os.RemoveAll(testDir) }()
+	_ = os.RemoveAll(testDir)
+
+	c := New(testDir)
+	defer func() { _ = c.Clear() }()
+
+	// Update DB config values
+	dbConn.Table("w_system_configs").Where("key = ?", "disk_cache_max_size_mb").Update("value", "250")
+	dbConn.Table("w_system_configs").Where("key = ?", "disk_cache_ttl_minutes").Update("value", "120")
+	dbConn.Table("w_system_configs").Where("key = ?", "disk_cache_lru_enabled").Update("value", "false")
+
+	// Invalidate Redis config cache to force DB reload
+	if cache.Redis != nil {
+		cache.Redis.Del(context.Background(), cache.PrefixedKey("system_configs"))
+	}
+
+	// Reload config
+	c.ReloadConfig(context.Background())
+
+	status := c.Status()
+	if status.MaxSizeMB != 250 {
+		t.Errorf("expected MaxSizeMB to be 250, got %d", status.MaxSizeMB)
+	}
+	if status.TTLMinutes != 120 {
+		t.Errorf("expected TTLMinutes to be 120, got %d", status.TTLMinutes)
+	}
+	if status.LRUEnabled != false {
+		t.Errorf("expected LRUEnabled to be false, got %v", status.LRUEnabled)
+	}
+}