feat: add Mihomo API client with refresh polling

Implements MihomoClient wrapping Mihomo's REST API with provider refresh
polling, config reload, and startup readiness check. Also fixes a
Python 3.14 + respx 0.21.1 compatibility issue where the httpcore mocker
passes bytes method to httpx.Request causing Method pattern matching to
fail; resolved by switching respx DEFAULT_MOCKER to the httpx-level mocker.

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

app/tests/conftest.py

@@ -2,6 +2,14 @@ import sys
 import os
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
 
+import respx.mocks
+# Python 3.14 + httpx 0.28 compatibility: httpcore passes method as bytes,
+# but the httpcore mocker reconstructs httpx.Request with bytes method,
+# causing Method pattern comparison ('GET' == b'GET') to always fail.
+# Use the httpx-level mocker instead, which patches AsyncClient._transport_for_url
+# and avoids the bytes/str mismatch entirely.
+respx.mocks.DEFAULT_MOCKER = "httpx"
+
 import pytest_asyncio
 from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker
 from models import Base