feat: route plugin HTTP traffic through SOCKS5 proxy

Add PySocks dependency and shared src/derp/http.py module providing
proxy-aware urlopen() and build_opener() that route through
socks5h://127.0.0.1:1080. Subclassed SocksiPyHandler passes SSL
context through to HTTPS connections.

Swapped 14 external-facing plugins to use the proxied helpers.
Local-only traffic (SearXNG, raw DNS/TLS sockets) stays direct.
Updated test mocks in test_twitch and test_alert accordingly.

plugins/username.py

@@ -17,6 +17,7 @@ import urllib.request
 from concurrent.futures import ThreadPoolExecutor
 from dataclasses import dataclass
 
+from derp.http import urlopen as _urlopen
 from derp.plugin import command
 
 log = logging.getLogger(__name__)
@@ -94,7 +95,7 @@ def _http_get(url: str, timeout: int = _TIMEOUT) -> tuple[int, str]:
 
     req = urllib.request.Request(url, headers={"User-Agent": _USER_AGENT})
     try:
-        with urllib.request.urlopen(req, timeout=timeout, context=ctx) as resp:
+        with _urlopen(req, timeout=timeout, context=ctx) as resp:
             body = resp.read().decode("utf-8", errors="replace")
             return resp.status, body
     except urllib.error.HTTPError as exc: