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/core/contracts/database.go

@@ -0,0 +1,23 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package contracts defines unified service interfaces and DTOs for cross-plugin communication.
+package contracts
+
+import (
+	"context"
+
+	"gorm.io/gorm"
+)
+
+// DBService defines the standard contract for relational database access and multi-datasource routing.
+type DBService interface {
+	// GORM returns the underlying GORM database instance.
+	GORM() *gorm.DB
+
+	// DB returns the GORM database instance bound to the given context.
+	DB(ctx context.Context) *gorm.DB
+
+	// Named returns a named database connection if multiple data sources or replicas are configured.
+	Named(name string) *gorm.DB
+}