build: add deps and test scaffolding for auto-reverse

Adds runtime deps (playwright, mitmproxy, anthropic, genson) and dev
deps (pytest, pytest-asyncio); creates the tests/ scaffold with
fixture_site.py, conftest.py, and a smoke test. Documents the
mitmproxy aioquic/mitmproxy-rs stub workaround needed for
free-threaded CPython 3.14.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_smoke.py

@@ -0,0 +1,16 @@
+"""Smoke test: verify the test infrastructure itself works."""
+
+from __future__ import annotations
+
+import urllib.request
+
+from tests.fixture_site import start_fixture_site
+
+
+def test_fixture_site_reachable() -> None:
+    server, base_url = start_fixture_site()
+    try:
+        with urllib.request.urlopen(f"{base_url}/api/users") as resp:
+            assert resp.status == 200
+    finally:
+        server.shutdown()