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/exports.go

@@ -0,0 +1,126 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package upload
+
+import (
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/cache"
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/filesrv"
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/handler"
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/ingest"
+	uploadstats "github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/stats"
+	uploadtask "github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/task"
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/util"
+	"github.com/Rain-kl/Wavelet/backend/plugins/drivers/driver_asynq_worker"
+)
+
+// HTTP handlers
+var (
+	UploadFile             = handler.UploadFile
+	DownloadFile           = handler.DownloadFile
+	BatchDownloadFiles     = handler.BatchDownloadFiles
+	ListFiles              = handler.ListFiles
+	DeleteFile             = handler.DeleteFile
+	GetDistinctUploadTypes = handler.GetDistinctUploadTypes
+	ListMyFiles            = handler.ListMyFiles
+	DeleteMyFile           = handler.DeleteMyFile
+	UpdateMyFile           = handler.UpdateMyFile
+	GetFileStats           = handler.GetFileStats
+	ServeFileByID          = filesrv.ServeFileByID
+)
+
+// Programmatic ingest API
+var (
+	Ingest      = ingest.Ingest
+	Remove      = ingest.Remove
+	RemoveOwned = ingest.RemoveOwned
+	FindByHash  = ingest.FindByHash
+)
+
+// Ingest policy constants
+const (
+	PolicyCreate          = ingest.PolicyCreate
+	PolicyDedupNewRecord  = ingest.PolicyDedupNewRecord
+	PolicyResolveExisting = ingest.PolicyResolveExisting
+)
+
+type (
+	// IngestRequest is the programmatic upload ingest payload.
+	IngestRequest = ingest.Request
+	// IngestResult reports ingest side effects.
+	IngestResult = ingest.Result
+	// IngestPolicy controls hash-collision behavior during ingest.
+	IngestPolicy = ingest.Policy
+)
+
+// Ingest errors
+var (
+	ErrIngestForbidden       = ingest.ErrForbidden
+	ErrIngestStorageReadOnly = ingest.ErrStorageReadOnly
+)
+
+// Cache management
+var (
+	ResetAccessCaches              = cache.ResetAccessCaches
+	PublishAccessCacheInvalidation = cache.PublishAccessCacheInvalidation
+)
+
+// Stats
+var (
+	// Deprecated: use upload.Ingest or upload.Remove; stats are applied internally.
+	ApplyUploadStatsAdd = uploadstats.ApplyUploadStatsAdd
+	// Deprecated: use upload.Ingest or upload.Remove; stats are applied internally.
+	ApplyUploadStatsRemove = uploadstats.ApplyUploadStatsRemove
+	RebuildUploadStats     = uploadstats.RebuildUploadStats
+)
+
+// Utilities
+var (
+	CompressImageToWebP = util.CompressImageToWebP
+	ValidateS3Key       = util.ValidateS3Key
+)
+
+// Task identifiers and metadata
+const (
+	StorageMigrationTask   = uploadtask.StorageMigrationTask
+	SystemCleanupTask      = uploadtask.SystemCleanupTask
+	WarmImageCacheTask     = uploadtask.WarmImageCacheTask
+	RebuildUploadStatsTask = uploadtask.RebuildUploadStatsTask
+)
+
+var (
+	// StorageMigrationMeta describes the storage migration async task.
+	StorageMigrationMeta = uploadtask.StorageMigrationMeta
+	// SystemCleanupMeta describes the orphaned upload cleanup task.
+	SystemCleanupMeta = uploadtask.SystemCleanupMeta
+	// WarmImageCacheMeta describes the image compression cache warmup task.
+	WarmImageCacheMeta = uploadtask.WarmImageCacheMeta
+	// RebuildUploadStatsMeta describes the upload stats rebuild task.
+	RebuildUploadStatsMeta = uploadtask.RebuildUploadStatsMeta
+)
+
+// MigrationHandler executes storage migration tasks.
+type MigrationHandler = uploadtask.MigrationHandler
+
+// SystemCleanupHandler removes orphaned upload files.
+type SystemCleanupHandler = uploadtask.SystemCleanupHandler
+
+// WarmImageCacheHandler pre-warms compressed image caches.
+type WarmImageCacheHandler = uploadtask.WarmImageCacheHandler
+
+// RebuildUploadStatsHandler rebuilds upload stats from active records.
+type RebuildUploadStatsHandler = uploadtask.RebuildUploadStatsHandler
+
+// WarmImageCachePayload is the payload for image cache warmup tasks.
+type WarmImageCachePayload = uploadtask.WarmImageCachePayload
+
+// Ensure task handler types implement required interfaces.
+var (
+	_ driver_asynq_worker.TaskHandler = (*MigrationHandler)(nil)
+	_ driver_asynq_worker.TaskHandler = (*SystemCleanupHandler)(nil)
+	_ driver_asynq_worker.TaskHandler = (*RebuildUploadStatsHandler)(nil)
+	_ interface {
+		driver_asynq_worker.TaskHandler
+		ValidatePayload([]byte) ([]byte, error)
+	} = (*WarmImageCacheHandler)(nil)
+)