chore(infra/cfn): add Forgejo network CloudFormation resources

Add EFS MountTargets (two subnets), an ALB Target Group for Forgejo (port 3000, health
checks), and a Listener Rule for host git.n-daisuke897.com; export the TargetGroup ARN. No
application-level configuration changes.

infra/cfn/forgejo-network.yaml

@@ -77,3 +77,11 @@ Resources:
           - "git.n-daisuke897.com"
         Field: "host-header"
       Transforms: []
+
+Outputs:
+
+  TargetGroupArn:
+    Description: ARN of the Forgejo target group
+    Value: !Ref ElasticLoadBalancingV2TargetGroupForgejo
+    Export:
+      Name: !Sub "${AWS::StackName}-TargetGroupArn"