feat: async HTTP client and parallel source fetching

Replace blocking urllib with a minimal async HTTP/1.1 client (http.py)
using asyncio streams. Pool source fetches now run in parallel via
asyncio.gather. Dead proxy reporting uses async POST. Handles
Content-Length, chunked transfer-encoding, and connection-close bodies.
No new dependencies.

tests/test_pool.py

@@ -1,6 +1,5 @@
 """Tests for the managed proxy pool."""
 
-import json
 import time
 
 import pytest
@@ -304,21 +303,20 @@ class TestProxyPoolReport:
             asyncio.run(pool._run_health_tests())
             mock_report.assert_not_called()
 
-    def test_report_sync_payload(self):
-        from unittest.mock import MagicMock, patch
+    def test_report_async_payload(self):
+        import asyncio
+        from unittest.mock import AsyncMock, patch
 
         cfg = ProxyPoolConfig(sources=[], report_url="http://api:8081/report")
         pool = ProxyPool(cfg, [], timeout=10.0)
 
-        dead = [{"proto": "socks5", "proxy": "10.0.0.1:1080"}]
-        with patch("s5p.pool.urllib.request.urlopen", new_callable=MagicMock) as mock_open:
-            mock_open.return_value.__enter__ = MagicMock()
-            mock_open.return_value.__exit__ = MagicMock(return_value=False)
-            pool._report_sync(dead)
-            req = mock_open.call_args[0][0]
-            assert req.method == "POST"
-            assert req.full_url == "http://api:8081/report"
-            assert b'"dead"' in req.data
+        with patch("s5p.pool.http_post_json", new_callable=AsyncMock) as mock_post:
+            asyncio.run(pool._report_dead(["socks5://10.0.0.1:1080"]))
+            mock_post.assert_called_once()
+            url = mock_post.call_args[0][0]
+            payload = mock_post.call_args[0][1]
+            assert url == "http://api:8081/report"
+            assert payload == {"dead": [{"proto": "socks5", "proxy": "10.0.0.1:1080"}]}
 
 
 class TestProxyPoolStaleExpiry: