[mod] limiter: trusted proxies (#4911)

Replaces `x_for` functionality with `trusted_proxies`. This allows defining
which IP / ranges to trust extracting the client IP address from X-Forwarded-For
and X-Real-IP headers.

We don't know if the proxy chain will give us the proper client
address (REMOTE_ADDR in the WSGI environment), so we rely on reading the headers
of the proxy before SearXNG (if there is one, in that case it must be added to
trusted_proxies) hoping it has done the proper checks. In case a proxy in the
chain does not check the client address correctly, integrity is compromised and
this should be fixed by whoever manages the proxy, not us.

Closes:

- https://github.com/searxng/searxng/issues/4940
- https://github.com/searxng/searxng/issues/4939
- https://github.com/searxng/searxng/issues/4907
- https://github.com/searxng/searxng/issues/3632
- https://github.com/searxng/searxng/issues/3191
- https://github.com/searxng/searxng/issues/1237

Related:

- https://github.com/searxng/searxng-docker/issues/386
- https://github.com/inetol-infrastructure/searxng-container/issues/81

searx/valkeydb.py

@@ -17,6 +17,7 @@ A valkey DB connect can be tested by::
   >>>
 
 """
+from __future__ import annotations
 
 import os
 import pwd
@@ -26,12 +27,12 @@ import warnings
 import valkey
 from searx import get_setting
 
-
-_CLIENT = None
+_CLIENT: valkey.Valkey | None = None
 logger = logging.getLogger(__name__)
 
 
-def client() -> valkey.Valkey:
+def client() -> valkey.Valkey | None:
+    """Returns SearXNG's global Valkey DB connector (Valkey client object)."""
     return _CLIENT