Swap PyPI download badge to pepy-backed endpoint

shields.io/pypi/dm has been returning "rate limited by upstream service"
with no ETA for a fix (badges/shields#11620). Switching to shields.io/pepy/dt
keeps the same visual style but uses pepy.tech as the data source.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: prmoore77

README.md

@@ -4,7 +4,7 @@
 [<img src="https://img.shields.io/badge/dockerhub-image-green?logo=Docker">](https://hub.docker.com/r/gizmodata/flight-sql-websocket-proxy)
 [![flight-sql-websocket-proxy-ci](https://github.com/gizmodata/flight-sql-websocket-proxy/actions/workflows/ci.yml/badge.svg)](https://github.com/gizmodata/flight-sql-websocket-proxy/actions/workflows/ci.yml)
 [![PyPI version](https://badge.fury.io/py/flight-sql-websocket-proxy.svg)](https://badge.fury.io/py/flight-sql-websocket-proxy)
-[![PyPI Downloads](https://img.shields.io/pypi/dm/flight-sql-websocket-proxy.svg)](https://pypi.org/project/flight-sql-websocket-proxy/)
+[![PyPI Downloads](https://img.shields.io/pepy/dt/flight-sql-websocket-proxy.svg)](https://pypi.org/project/flight-sql-websocket-proxy/)
 
 An Arrow Flight SQL WebSocket Proxy Server