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/cache.go

@@ -0,0 +1,32 @@
+// 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"
+	"errors"
+	"time"
+)
+
+// ErrCacheMiss is returned when an item is not found in the cache.
+var ErrCacheMiss = errors.New("contracts/cache: key not found")
+
+// CacheService defines the contract for multi-layer cache operations (RAM L1 + Redis L2 + Pub/Sub invalidation).
+type CacheService interface {
+	// Get retrieves an item from cache into target. Returns ErrCacheMiss if not found.
+	Get(ctx context.Context, key string, target any) error
+
+	// Set stores an item into cache with a specified time-to-live duration.
+	Set(ctx context.Context, key string, value any, ttl time.Duration) error
+
+	// Delete evicts a key from local and remote cache tiers and broadcasts invalidation.
+	Delete(ctx context.Context, key string) error
+
+	// GetOrSet retrieves an item from cache, or calls loader to populate and return if missing.
+	GetOrSet(ctx context.Context, key string, target any, ttl time.Duration, loader func() (any, error)) error
+
+	// Invalidate is a semantic alias for Delete.
+	Invalidate(ctx context.Context, key string) error
+}