feat: add per-proxy backoff after connection failure

Track last_fail timestamp on ProxyEntry. When a connection attempt fails
in server.py, report_failure() records the time. The selection weight
multiplies by min(fail_age/60, 1.0), ramping back from floor over 60s.
Prevents wasting retries on proxies that just failed.

tests/test_pool.py

@@ -116,6 +116,42 @@ class TestProxyPoolWeight:
         entry.last_ok = 0
         assert ProxyPool._weight(entry, now) == pytest.approx(0.01)
 
+    def test_weight_failure_penalty(self):
+        hop = ChainHop(proto="socks5", host="1.2.3.4", port=1080)
+        now = 1000.0
+
+        # healthy proxy: weight ~1.0
+        entry = ProxyEntry(hop=hop, last_ok=now)
+        assert ProxyPool._weight(entry, now) == pytest.approx(1.0)
+
+        # just failed: weight drops to floor (fail_age=0 -> multiplier=0)
+        entry.last_fail = now
+        assert ProxyPool._weight(entry, now) == pytest.approx(0.01)
+
+        # 30s after failure: base=1/(1+30/300)=0.909, penalty=30/60=0.5 -> ~0.45
+        assert ProxyPool._weight(entry, now + 30) == pytest.approx(0.4545, abs=0.05)
+
+        # 60s after failure: penalty=1.0, only base decay -> 1/(1+60/300)=0.833
+        assert ProxyPool._weight(entry, now + 60) == pytest.approx(0.833, abs=0.05)
+
+    def test_report_failure(self):
+        cfg = ProxyPoolConfig(sources=[])
+        pool = ProxyPool(cfg, [], timeout=10.0)
+        hop = ChainHop(proto="socks5", host="1.2.3.4", port=1080)
+        pool._proxies["socks5://1.2.3.4:1080"] = ProxyEntry(
+            hop=hop, alive=True, last_ok=time.time(),
+        )
+
+        assert pool._proxies["socks5://1.2.3.4:1080"].last_fail == 0.0
+        pool.report_failure(hop)
+        assert pool._proxies["socks5://1.2.3.4:1080"].last_fail > 0.0
+
+    def test_report_failure_unknown_proxy(self):
+        cfg = ProxyPoolConfig(sources=[])
+        pool = ProxyPool(cfg, [], timeout=10.0)
+        hop = ChainHop(proto="socks5", host="9.9.9.9", port=1080)
+        pool.report_failure(hop)  # should not raise
+
 
 class TestProxyPoolFetchFile:
     """Test file source parsing."""