Mercure 0.14 configuration behind Nginx reverse proxy with supervisor - server not responding

I need help configuring Mercure 0.14 to run on port 3000 on a production server, which nginx will proxy to behind the path /mercure/.

To install Mercure, I did the following:

1. Grabbed Mercure 0.14 linux x86_64 binary from the release page on github. https://github.com/dunglas/mercure/releases
2. Unpacked the tarball and moved the mercure binary to /usr/bin/mercure

I set up supervisor with the following configuration (with sensitive info redacted):

    # file: /etc/supervisor/conf.d/mercure.conf
    [program:mercure]
    command=/usr/bin/mercure run
    process_name=%(program_name)s_%(process_num)s
    numprocs=1
    environment=HOME="/home/[redacted]",USER="[redacted]",JWT_KEY=[redacted],SUBSCRIPTIO\
NS="1",CORS_ALLOWED_ORIGINS="[redacted]",USE_FORWARDED_HEADERS="1",PUBLISH_ALLOWED_ORIGIN\
S="http://localhost:3000",directory=/tmp,SERVER_NAME=:3000
    autostart=true
    autorestart=true
    startsecs=5
    startretries=10
    user=[redacted]
    redirect_stderr=false
    stdout_capture_maxbytes=1MB
    stderr_capture_maxbytes=1MB
    stdout_logfile=/var/log/mercure.out.log
    stderr_logfile=/var/log/mercure.error.log

Note that I set SERVER_NAME=:3000 in order to run it on port 3000.

Supervisor was able to start the process and shows it as running.

$ sudo supervisorctl status
mercure:mercure_0                RUNNING   pid 1410254, uptime 0:09:30

However, I'm not able to curl the server...

$ curl localhost:3000/.well-known/mercure
curl: (7) Failed to connect to localhost port 3000: Connection refused

Finally, I have Nginx set up to proxy requests on port 80 to the mercure server on port 3000. This is my Nginx configuration:

    location /mercure/ {
        proxy_pass http://127.0.0.1:3000/;
        proxy_read_timeout 24h;
        proxy_http_version 1.1;
        proxy_set_header Connection "";
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Host $host;
        proxy_set_header X-Forwarded-Proto $scheme;
    }

However, going to (servername)/mercure/.well-known/mercure results in a 502.

Mercure's logs only show:

{"level":"info","ts":1675706204.7100313,"logger":"admin","msg":"admin endpoint started","address":"localhost:2019","enforce_origin":false,"origins":["//localhost:2019","//[::1]:2019","//127.0.0.1:2019"]}
{"level":"info","ts":1675706204.7100613,"msg":"serving initial configuration"}

But there is no error message in the logs.

I think the Nginx part is correct, but for some reason the server is just not running/responding on port 3000. Can anyone help?