fix: centralize retry logic in proxy transport layer

Add exponential-backoff retry (3 attempts) for transient SSL,
connection, timeout, and OS errors to all three proxy functions:
urlopen, create_connection, open_connection. Remove per-plugin
retry from alert.py since transport layer now handles it.

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

plugins/alert.py

@@ -5,7 +5,6 @@ from __future__ import annotations
 import asyncio
 import json
 import re
-import ssl
 import urllib.request
 from datetime import datetime, timezone
 
@@ -272,22 +271,11 @@ async def _poll_once(bot, key: str, announce: bool = True) -> None:
     loop = asyncio.get_running_loop()
 
     for tag, backend in _BACKENDS.items():
-        items = None
-        for attempt in range(3):
-            try:
-                items = await loop.run_in_executor(None, backend, keyword)
-                break
-            except (ssl.SSLError, ConnectionError, TimeoutError, OSError) as exc:
-                if attempt < 2:
-                    await asyncio.sleep(2 ** attempt)
-                    continue
-                data["last_error"] = f"{tag}: {exc}"
-                had_error = True
-            except Exception as exc:
-                data["last_error"] = f"{tag}: {exc}"
-                had_error = True
-                break
-        if items is None:
+        try:
+            items = await loop.run_in_executor(None, backend, keyword)
+        except Exception as exc:
+            data["last_error"] = f"{tag}: {exc}"
+            had_error = True
             continue
 
         seen_set = set(data.get("seen", {}).get(tag, []))