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/admin/handlers_user.go

@@ -247,7 +247,7 @@ func DeleteUser(c *gin.Context) {
 		return
 	}
 
-	currUser, _ := auth.GetFromContext[*contracts.UserDTO](c, auth.UserObjKey)
+	currUser, _ := util.GetFromContext[*contracts.UserDTO](c, contracts.AuthUserObjKey)
 	if currUser == nil {
 		response.AbortUnauthorized(c, AdminRequired)
 		return
@@ -338,7 +338,7 @@ func UpdateUser(c *gin.Context) {
 		return
 	}
 
-	currUser, _ := auth.GetFromContext[*contracts.UserDTO](c, auth.UserObjKey)
+	currUser, _ := util.GetFromContext[*contracts.UserDTO](c, contracts.AuthUserObjKey)
 	if currUser == nil {
 		response.AbortUnauthorized(c, AdminRequired)
 		return