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/logger.go

@@ -0,0 +1,39 @@
+// 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"
+)
+
+// LoggerService defines the contract for structured logging with trace ID and context correlation.
+type LoggerService interface {
+	// Debug logs a debug message with optional key-value structured fields.
+	Debug(ctx context.Context, msg string, keysAndValues ...any)
+
+	// Info logs an informational message with optional key-value structured fields.
+	Info(ctx context.Context, msg string, keysAndValues ...any)
+
+	// Warn logs a warning message with optional key-value structured fields.
+	Warn(ctx context.Context, msg string, keysAndValues ...any)
+
+	// Error logs an error message with optional key-value structured fields.
+	Error(ctx context.Context, msg string, keysAndValues ...any)
+
+	// Debugf logs a formatted debug message.
+	Debugf(ctx context.Context, format string, args ...any)
+
+	// Infof logs a formatted informational message.
+	Infof(ctx context.Context, format string, args ...any)
+
+	// Warnf logs a formatted warning message.
+	Warnf(ctx context.Context, format string, args ...any)
+
+	// Errorf logs a formatted error message.
+	Errorf(ctx context.Context, format string, args ...any)
+
+	// With returns a child logger enriched with additional key-value attributes.
+	With(keysAndValues ...any) LoggerService
+}