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/events.go

@@ -0,0 +1,109 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package contracts defines unified service interfaces and DTOs for cross-plugin communication.
+package contracts
+
+// ======================================================================
+// Domain Event Topic Constants
+// ======================================================================
+//
+// All cross-plugin domain event topics MUST be declared here so that
+// producers and consumers share the same string values without importing
+// each other's implementation packages.
+// ======================================================================
+
+// --- Auth & User Events ---
+const (
+	// EventTopicAdminLoggedIn fires when an admin user logs in.
+	EventTopicAdminLoggedIn = "admin:logged_in"
+
+	// EventTopicUserCreated fires when a new user account is created.
+	EventTopicUserCreated = "user:created"
+
+	// EventTopicUserUpdated fires when a user profile is updated.
+	EventTopicUserUpdated = "user:updated"
+
+	// EventTopicUserDeleted fires when a user account is deleted.
+	EventTopicUserDeleted = "user:deleted"
+)
+
+// --- Admin & System Events ---
+const (
+	// EventTopicConfigChanged fires when a system configuration value changes.
+	EventTopicConfigChanged = "admin:config_changed"
+
+	// EventTopicSystemCleanup fires when a periodic system cleanup completes.
+	EventTopicSystemCleanup = "admin:system_cleanup"
+)
+
+// --- Upload / Storage Events ---
+const (
+	// EventTopicUploadCreated fires when a new file upload is recorded.
+	EventTopicUploadCreated = "upload:created"
+
+	// EventTopicUploadDeleted fires when a file upload is removed.
+	EventTopicUploadDeleted = "upload:deleted"
+
+	// EventTopicIngestComplete fires when a programmatic file ingest finishes.
+	EventTopicIngestComplete = "upload:ingest_complete"
+)
+
+// --- Message Gateway Events ---
+const (
+	// EventTopicNotificationSent fires when a push notification is dispatched.
+	EventTopicNotificationSent = "message:notification_sent"
+
+	// EventTopicChannelBound fires when a user binds a messaging channel.
+	EventTopicChannelBound = "message:channel_bound"
+
+	// EventTopicChannelUnbound fires when a user unbinds a messaging channel.
+	EventTopicChannelUnbound = "message:channel_unbound"
+)
+
+// --- Risk Control Events ---
+const (
+	// EventTopicAccessLogRecorded fires when a user access log entry is recorded.
+	EventTopicAccessLogRecorded = "risk:access_log_recorded"
+)
+
+// ======================================================================
+// Domain Event Payload DTOs
+// ======================================================================
+
+// AdminLoggedIn 管理员登录领域事件载荷
+type AdminLoggedIn struct {
+	User *UserDTO `json:"user"`
+	IP   string   `json:"ip"`
+}
+
+// UserCreatedEvent fires when a new user account is created.
+type UserCreatedEvent struct {
+	User     *UserDTO `json:"user"`
+	Password string   `json:"-"`
+}
+
+// ConfigChangedEvent fires when a system configuration value changes.
+type ConfigChangedEvent struct {
+	Key    string `json:"key"`
+	OldVal any    `json:"old_val,omitempty"`
+	NewVal any    `json:"new_val,omitempty"`
+}
+
+// UploadCreatedEvent fires when a new file upload is recorded.
+type UploadCreatedEvent struct {
+	UploadID uint64 `json:"upload_id,string"`
+	UserID   uint64 `json:"user_id,string"`
+	FileName string `json:"file_name"`
+	FileSize int64  `json:"file_size"`
+	MimeType string `json:"mime_type"`
+}
+
+// NotificationSentEvent fires when a push notification is dispatched.
+type NotificationSentEvent struct {
+	UserID    uint64 `json:"user_id,string"`
+	Channel   string `json:"channel"`
+	Title     string `json:"title"`
+	Success   bool   `json:"success"`
+	ErrorInfo string `json:"error_info,omitempty"`
+}