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/cache/errs.go

@@ -0,0 +1,12 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package cache
+
+const (
+	errRedisHashSetFailed    = "failed to set redis hash: %w"
+	errRedisHashDeleteFailed = "failed to delete redis hash field: %w"
+	errUnmarshalDataFailed   = "failed to unmarshal data: %w"
+	errMarshalDataFailed     = "failed to marshal data: %w"
+	errRedisKeySetFailed     = "failed to set redis key: %w"
+)