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>

src/derp/http.py

@@ -1,8 +1,10 @@
 """Proxy-aware HTTP/TCP helpers -- routes outbound traffic through SOCKS5."""
 
 import asyncio
+import logging
 import socket
 import ssl
+import time
 import urllib.request
 
 import socks
@@ -11,6 +13,10 @@ from sockshandler import SocksiPyConnectionS, SocksiPyHandler
 
 _PROXY_ADDR = "127.0.0.1"
 _PROXY_PORT = 1080
+_MAX_RETRIES = 3
+_RETRY_ERRORS = (ssl.SSLError, ConnectionError, TimeoutError, OSError)
+
+_log = logging.getLogger(__name__)
 
 
 class _ProxyHandler(SocksiPyHandler, urllib.request.HTTPSHandler):
@@ -35,13 +41,25 @@ class _ProxyHandler(SocksiPyHandler, urllib.request.HTTPSHandler):
 
 
 def urlopen(req, *, timeout=None, context=None):
-    """Proxy-aware drop-in for urllib.request.urlopen."""
+    """Proxy-aware drop-in for urllib.request.urlopen.
+
+    Retries on transient SSL/connection errors with exponential backoff.
+    """
     handler = _ProxyHandler(context=context)
     opener = urllib.request.build_opener(handler)
     kwargs = {}
     if timeout is not None:
         kwargs["timeout"] = timeout
-    return opener.open(req, **kwargs)
+    for attempt in range(_MAX_RETRIES):
+        try:
+            return opener.open(req, **kwargs)
+        except _RETRY_ERRORS as exc:
+            if attempt + 1 >= _MAX_RETRIES:
+                raise
+            delay = 2 ** attempt
+            _log.debug("urlopen retry %d/%d after %s: %s",
+                       attempt + 1, _MAX_RETRIES, type(exc).__name__, exc)
+            time.sleep(delay)
 
 
 def build_opener(*handlers, context=None):
@@ -54,20 +72,31 @@ def create_connection(address, *, timeout=None):
     """SOCKS5-proxied drop-in for socket.create_connection.
 
     Returns a connected socksocket (usable as context manager).
+    Retries on transient connection errors with exponential backoff.
     """
     host, port = address
-    sock = socks.socksocket()
-    sock.set_proxy(SOCKS5, _PROXY_ADDR, _PROXY_PORT, rdns=True)
-    if timeout is not None:
-        sock.settimeout(timeout)
-    sock.connect((host, port))
-    return sock
+    for attempt in range(_MAX_RETRIES):
+        try:
+            sock = socks.socksocket()
+            sock.set_proxy(SOCKS5, _PROXY_ADDR, _PROXY_PORT, rdns=True)
+            if timeout is not None:
+                sock.settimeout(timeout)
+            sock.connect((host, port))
+            return sock
+        except _RETRY_ERRORS as exc:
+            if attempt + 1 >= _MAX_RETRIES:
+                raise
+            delay = 2 ** attempt
+            _log.debug("create_connection retry %d/%d after %s: %s",
+                       attempt + 1, _MAX_RETRIES, type(exc).__name__, exc)
+            time.sleep(delay)
 
 
 async def open_connection(host, port, *, timeout=None):
     """SOCKS5-proxied drop-in for asyncio.open_connection.
 
     SOCKS5 handshake runs in a thread executor; returns (reader, writer).
+    Retries on transient connection errors with exponential backoff.
     """
     loop = asyncio.get_running_loop()
 
@@ -80,5 +109,14 @@ async def open_connection(host, port, *, timeout=None):
         sock.setblocking(False)
         return sock
 
-    sock = await loop.run_in_executor(None, _connect)
-    return await asyncio.open_connection(sock=sock)
+    for attempt in range(_MAX_RETRIES):
+        try:
+            sock = await loop.run_in_executor(None, _connect)
+            return await asyncio.open_connection(sock=sock)
+        except _RETRY_ERRORS as exc:
+            if attempt + 1 >= _MAX_RETRIES:
+                raise
+            delay = 2 ** attempt
+            _log.debug("open_connection retry %d/%d after %s: %s",
+                       attempt + 1, _MAX_RETRIES, type(exc).__name__, exc)
+            await asyncio.sleep(delay)