feat(proxy): refactor proxy configuration and add DataImpulse provider

- Updated proxy configuration in `.env.example` files to use `PROXY_URL` and `PROXY_URLS` instead of `CUSTOM_PROXY_URL` and `CUSTOM_PROXY_URLS`.
- Introduced `DataImpulseProvider` for proxy management, replacing the deprecated `AnonymousProxiesProvider`.
- Enhanced documentation to reflect changes in proxy setup and usage.
- Adjusted related code in the proxy registry and configuration files to support the new provider structure.

surfsense_backend/app/utils/proxy/providers/dataimpulse.py

@@ -0,0 +1,56 @@
+"""DataImpulse residential / rotating proxy provider.
+
+Takes the shared ``PROXY_URL`` env, exactly like the BYO ``custom`` provider —
+the format is uniform, paste it straight from the vendor dashboard. What makes
+this a *named* provider rather than just ``custom`` is the vendor-specific
+knowledge it layers on top of that URL:
+
+* :meth:`get_location` reads DataImpulse's ``__cr.<country>`` username suffix so
+  the crawler's geoip-match can align the browser locale with the exit country
+  (``custom`` can't — it treats the URL as opaque).
+
+Rotation happens server-side (a fresh exit IP per request on the default pool
+port), so this is NOT :pyattr:`~ProxyProvider.is_pool_backed`.
+
+Example URL::
+
+    http://<token>__cr.us:<password>@gw.dataimpulse.com:823
+
+ponytail: sticky sessions (a stable exit IP across requests) are another
+username suffix (``__sid.<id>``) — the lever the Reddit scraper's README flags as
+a TODO for its ``loid``-per-IP flow. Not built yet: Reddit isn't wired to a
+route, so there's no caller to thread a session id through. Add a
+``get_sticky_proxy_url(session_id)`` here (rewriting the username) when it lands.
+"""
+
+import logging
+from urllib.parse import urlsplit
+
+from app.config import Config
+from app.utils.proxy.base import ProxyProvider
+
+logger = logging.getLogger(__name__)
+
+# DataImpulse encodes country routing as a "__cr.<country>" username suffix; the
+# country token runs until the next "__" param (e.g. "__sid") or the end.
+_COUNTRY_MARKER = "__cr."
+
+
+class DataImpulseProvider(ProxyProvider):
+    """Provider for a DataImpulse proxy URL in the shared ``PROXY_URL`` env."""
+
+    name = "dataimpulse"
+
+    def get_proxy_url(self) -> str | None:
+        url = (Config.PROXY_URL or "").strip()
+        return url or None
+
+    def get_location(self) -> str:
+        """Country parsed from the ``__cr.<country>`` username suffix, or ``""``."""
+        url = self.get_proxy_url()
+        if not url:
+            return ""
+        username = urlsplit(url).username or ""
+        if _COUNTRY_MARKER not in username:
+            return ""
+        return username.split(_COUNTRY_MARKER, 1)[1].split("__", 1)[0].lower()