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/plugin_test.go

@@ -133,7 +133,7 @@ func TestAuthPluginUnit(t *testing.T) {
 	require.NoError(t, authSvc.RevokeUserSessions(context.Background(), user.ID))
 
 	// GetCurrentUser from context
-	userCtx := context.WithValue(context.Background(), auth.UserObjKey, userDTO)
+	userCtx := context.WithValue(context.Background(), contracts.AuthUserObjKey, userDTO)
 	current, err := authSvc.GetCurrentUser(userCtx)
 	require.NoError(t, err)
 	assert.Equal(t, user.ID, current.ID)