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

Question

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:
Grabbed Mercure 0.14 linux x86_64 binary from the release page on github. https://github.com/dunglas/mercure/releases
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?