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/ingest/ingest.go

@@ -0,0 +1,64 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package ingest
+
+import (
+	"context"
+	"errors"
+
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/models"
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/repository"
+	"gorm.io/gorm"
+)
+
+// Ingest stores or resolves an upload using the configured policy and side effects.
+func Ingest(ctx context.Context, req Request) (Result, error) {
+	normalizeRequest(&req)
+	if req.Hash == "" {
+		return Result{}, errors.New("ingest hash is required")
+	}
+	if req.Reader == nil {
+		return Result{}, errors.New("ingest reader is required")
+	}
+	if req.Size < 0 {
+		return Result{}, errors.New("ingest size must be non-negative")
+	}
+
+	switch req.Policy {
+	case PolicyDedupNewRecord, PolicyResolveExisting:
+		return ingestWithHashPolicy(ctx, req)
+	case PolicyCreate:
+		return createNewUpload(ctx, req)
+	default:
+		return Result{}, errors.New("unsupported ingest policy")
+	}
+}
+
+// FindByHash returns a reusable active upload with the same hash and size.
+func FindByHash(ctx context.Context, hash string, size int64) (models.Upload, error) {
+	return repository.FindReusableUploadByHash(ctx, hash, size)
+}
+
+func ingestWithHashPolicy(ctx context.Context, req Request) (Result, error) {
+	existing, err := repository.FindReusableUploadByHash(ctx, req.Hash, req.Size)
+	if err == nil {
+		switch req.Policy {
+		case PolicyResolveExisting:
+			return Result{
+				Upload:   existing,
+				Resolved: true,
+			}, nil
+		case PolicyDedupNewRecord:
+			if uploadstorageReadOnly(ctx) {
+				return Result{}, ErrStorageReadOnly
+			}
+			return createDedupRecord(ctx, existing, req)
+		}
+	}
+	if err != nil && !errors.Is(err, gorm.ErrRecordNotFound) {
+		return Result{}, err
+	}
+
+	return createNewUpload(ctx, req)
+}