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/handler/logics.go

@@ -0,0 +1,87 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package handler
+
+import (
+	"context"
+	"errors"
+	"sort"
+
+	"gorm.io/gorm"
+
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/ingest"
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/models"
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/repository"
+)
+
+func listUploadFiles(ctx context.Context, filter repository.UploadListFilter) (int64, []models.Upload, error) {
+	return repository.ListUploads(ctx, filter)
+}
+
+func listMyUploadFiles(ctx context.Context, userID uint64, filter repository.UploadListFilter) (int64, []models.Upload, error) {
+	filter.UserID = userID
+	return repository.ListUploads(ctx, filter)
+}
+
+func softDeleteUpload(ctx context.Context, uploadID uint64) (models.Upload, error) {
+	return ingest.Remove(ctx, uploadID)
+}
+
+func softDeleteOwnedUpload(ctx context.Context, userID, uploadID uint64) (models.Upload, error) {
+	return ingest.RemoveOwned(ctx, userID, uploadID)
+}
+
+func listDistinctUploadTypes(ctx context.Context) ([]string, error) {
+	types, err := repository.ListDistinctUploadTypes(ctx)
+	if err != nil {
+		return nil, err
+	}
+	sort.Strings(types)
+	return types, nil
+}
+
+type updateMyUploadInput struct {
+	FileName   string
+	AccessMode *int
+}
+
+func updateOwnedUpload(ctx context.Context, userID, uploadID uint64, input updateMyUploadInput) (models.Upload, error) {
+	u, err := repository.GetActiveUploadByID(ctx, uploadID)
+	if err != nil {
+		return models.Upload{}, err
+	}
+	if u.UserID != userID {
+		return models.Upload{}, ingest.ErrForbidden
+	}
+
+	updates := make(map[string]any)
+	if input.FileName != "" {
+		updates["file_name"] = input.FileName
+	}
+	if input.AccessMode != nil {
+		updates["access_mode"] = *input.AccessMode
+	}
+	if err := repository.UpdateUpload(ctx, &u, updates); err != nil {
+		return models.Upload{}, err
+	}
+	if name, ok := updates["file_name"].(string); ok {
+		u.FileName = name
+	}
+	if mode, ok := updates["access_mode"].(int); ok {
+		u.AccessMode = mode
+	}
+	return u, nil
+}
+
+func listUploadsForBatchDownload(ctx context.Context, ids []uint64) ([]models.Upload, error) {
+	return repository.ListUploadsByIDs(ctx, ids)
+}
+
+func loadUploadStats(ctx context.Context) ([]models.UploadStat, error) {
+	return repository.ListUploadStats(ctx)
+}
+
+func isRecordNotFound(err error) bool {
+	return errors.Is(err, gorm.ErrRecordNotFound)
+}