fix: rate-limit and audit by real client address

uvicorn does not trust forwarded headers from a peer outside
forwarded_allow_ips, so request.client.host was the web gateway for every
request. The auth-source bucket therefore counted all customers together:
60 failed logins from one attacker locked out everyone. Security events
recorded the gateway address, which made the audit trail useless for
attribution.

The gateway now overwrites X-Forwarded-For with the peer address it observed
instead of appending to whatever the client sent, so the header carries one
value the client cannot choose, and client_ip() resolves it with a fallback to
the connection peer.

The guest-session limiter was keyed on the environment name, making it one
global bucket of 120 per 15 minutes: roughly eight new visitors a minute for
the whole site before legitimate traffic started receiving 429. It is now per
source, and the ceiling is deliberately generous because offices and mobile
carriers put many real customers behind a single address.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

local/runtime_security_test.py

@@ -4,11 +4,30 @@ from unittest.mock import patch
 from botocore.exceptions import ClientError
 from .db import connect
 from .adapters import LocalS3Storage
-from .auth import password_hash, password_matches
+from .auth import client_ip, password_hash, password_matches
 from .scanning import ClamAV, require_clean
 from fastapi import HTTPException
 
+class FakeRequest:
+    def __init__(self, headers=None, peer='10.0.0.2'):
+        self.headers=headers or {}
+        self.client=type('C',(),{'host':peer})() if peer else None
+
+def check_client_ip():
+    """The gateway hands one address; a direct peer falls back to its own."""
+    # Gateway-set header wins over the connection peer, which is the gateway.
+    assert client_ip(FakeRequest({'x-forwarded-for':'198.51.100.9'}))=='198.51.100.9'
+    # Only the first entry is used, and it is length-capped.
+    assert client_ip(FakeRequest({'x-forwarded-for':'198.51.100.9, 10.0.0.2'}))=='198.51.100.9'
+    assert len(client_ip(FakeRequest({'x-forwarded-for':'a'*500})))<=64
+    # No header: the peer address, never a constant shared by every request.
+    assert client_ip(FakeRequest(peer='192.0.2.5'))=='192.0.2.5'
+    assert client_ip(FakeRequest({'x-forwarded-for':'   '},peer='192.0.2.5'))=='192.0.2.5'
+    assert client_ip(FakeRequest(peer=None))=='unknown'
+    print('PASS: client address resolution for rate-limit buckets and audit events')
+
 def run():
+    check_client_ip()
     with connect() as c:
         role=c.execute('SELECT rolsuper,rolcreatedb,rolcreaterole,rolbypassrls FROM pg_roles WHERE rolname=current_user').fetchone()
         assert not any(role.values()),role