fix: pin uvicorn loop to asyncio (loop=auto silently re-enables uvloop)

[ServerSideHannes]

Problem

#134 removed our uvloop.install() call, but pods still crashed with the identical libuv uv__io_poll abort (2026-07-18 10:18 UTC, faulthandler trace shows uvloop frames on a 2026.7.22 pod). Root cause: uvicorn's default loop="auto" installs uvloop itself whenever the package is importable — the #134 gate was a no-op in practice.

Change

• Pass loop="asyncio" in the uvicorn config unless S3PROXY_UVLOOP=1 (which uses loop="auto").
• Log the actually-running event loop class at startup (event_loop=... on the Starting line) so deploys can be verified against the runtime, not against which code path printed a log line.
• Tests assert the uvicorn config pins the loop by default and only relaxes to auto on explicit opt-in.