fix: disable uvloop by default (libuv abort under backend connection churn)

[ServerSideHannes]

Problem

s3proxy pods crash with exit 139 and no Python traceback under backup load. Faulthandler traces from today's three crashes (2026-07-16) all show the same thing: when the backend drops connections under load, an fd gets closed while uvloop is still polling it — libuv hits an assertion in uv__io_poll and calls abort(), killing every in-flight upload on the pod.

OSError: [Errno 9] Bad file descriptor
  uvloop/handles/tcp.pyx", line 50, in uvloop.loop.__tcp_get_socket
Fatal Python error: Aborted
  uvloop/loop.cpython-314-x86_64-linux-gnu.so: uv__io_poll+0x30d

uvloop 0.22.1, Python 3.14.6.

Change

Skip uvloop.install() unless S3PROXY_UVLOOP=1. Stock asyncio is slightly slower but has no native abort path.