refactor(core): fix cross-domain auth imports, unify migration, add downstream scaffold

Architecture:
- Move GetFromContext/SetToContext from plugins/domain/auth to pkg/util
- Move auth context key constants to core/contracts (AuthUserObjKey, AuthTokenAuthKey, etc.)
- Add AuthUserIDKey, AuthUserNameKey, GetCurrentUserID, RevokeToken to contracts.AuthService
- All 4 domain plugin Apply() methods now resolve AuthService via core.Using IoC
- Plugin route middleware uses authSvc.RequireAuthMiddleware() cast to gin.HandlerFunc
- DisallowTokenAuth added to AuthService contract

Migration:
- Replace cmd/app.go SetMigrationRunner bridge with gooseEngine implementing core.MigrationEngine
- Remove cmd/root.go PreRun migration hooks and runMigrations() function
- Migrations now run via core.App.Start() → RunMigrations()

Events:
- Add complete domain event topic catalog and payload DTOs to core/contracts/events.go
- 15 event topics across auth, user, admin, upload, message_gateway, risk_control

Downstream:
- Create downstream/ directory with README and custom_example plugin scaffold

CI:
- Update Makefile code-check architecture guards for Cordis layering
- Enforce: core no gin/gorm/asynq, contracts no plugins/, pkg no plugins/, domain no cross-domain

plugins/domain/auth/handlers.go

@@ -430,7 +430,7 @@ func handleCallbackRegister(ctx context.Context, c *gin.Context, source *AuthSou
 
 // UserInfo 获取当前登录用户信息
 func UserInfo(c *gin.Context) {
-	user, _ := GetFromContext[*contracts.UserDTO](c, UserObjKey)
+	user, _ := util.GetFromContext[*contracts.UserDTO](c, contracts.AuthUserObjKey)
 	session := sessions.Default(c)
 	needChange := session.Get("need_change_password") == true