feat: run with gunicorn instead of flask

Each worker now uses its own registry instead of the global REGISTRY
to avoid duplicate metric registration, since metrics are served
from the database.

run_gunicorn.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+flask db upgrade
+gunicorn "$FLASK_APP".__init__:app -w "$GUNICORN_WORKERS" --access-logfile=- -b "$GUNICORN_RUN_HOST:$GUNICORN_RUN_PORT"