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/errors.go

@@ -0,0 +1,16 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package ingest
+
+import (
+	"errors"
+
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/shared"
+)
+
+// ErrForbidden indicates the caller is not allowed to mutate the upload record.
+var ErrForbidden = errors.New("upload forbidden")
+
+// ErrStorageReadOnly indicates the storage backend is in migration read-only mode.
+var ErrStorageReadOnly = errors.New(shared.ErrStorageReadOnly)