Okay, I figured this out. The port was written as a site setting in the database and is not editable via /admin/site_settings. I had to:
$ cd /opt/<installdir>/
$ sudo ./use_discourse
$ psql bitnami_discourse
...to acces the database. Next I examined the site_settings table to find the offending parameter...
bitnami_discourse=# SELECT * FROM site_settings;
This told me that the "id" of the port row was 10 and the "value" was 8080. I needed to set this to NULL...
bitnami_discourse=# UPDATE site_settings SET value = NULL WHERE id = 10;
Next I restarted my stack...
$ sudo ./ctlscript.sh restart
I verified that new uploads had the correct URL but old ones did not. So finally, I rebaked my old posts:
$ cd /opt/<installdir/apps/discourse/scripts
$ sudo ./rebakeposts
All is well now. I hope this helps someone along the way.