Key the client address on the connection, not a header the caller sends

client_ip() read the first X-Forwarded-For hop unconditionally. The
login rate limit and the account lockout are keyed on that value and the
audit log records it, so rotating the header per request bought an
unbounded budget and an untraceable trail.

This walked around TRUSTED_PROXY_COUNT, which exists to decide exactly
this. The app leaves ProxyFix off by default and says why, ten lines of
comment away: ProxyFix trusts X-Forwarded-*, a client can send those
itself, and the login limiter is keyed on the address. Then client_ip
read the header anyway, so the stated protection never applied.

Resolution is now from remote_addr, which ProxyFix has already filled
from the header wherever a proxy is genuinely configured. Both cases end
at the same trustworthy source, and the port still comes off because ARR
forwards clientip:port and ProxyFix passes it through verbatim.

geenforce and tools had each hand-rolled the same mistake for their own
limiters. Rather than duplicate security-relevant logic a third time,
client_ip and strip_port join the shopdb.api surface (contract 0.28.0).
geenforce's _trusted_client_ip, which already refused the raw header for
its IP allowlist, is untouched and was the model for this.

The stability table was also missing rows for 0.26.0 and 0.27.0.

INSTALL-WINDOWS-IIS.md never mentioned TRUSTED_PROXY_COUNT, so a site
installed from it runs behind IIS with the setting at 0: every request
looks like it came from IIS, the whole site shares one rate-limit
bucket, and the audit log records the proxy for every action. It now
says so, and says not to set it when nothing is in front.

Three test modules simulated separate callers by setting
X-Forwarded-For, which is the header a real attacker rotates to escape
the limit. One of them, test_client_ip_prefers_forwarded_first_hop_
stripped, asserted the defect outright. The suite could not have caught
this. They now drive REMOTE_ADDR, with a regression guard that forging
the header buys nothing.

Scope: product

shopdb/api/__init__.py

@@ -104,6 +104,12 @@ def collector_authorized(pluginname: str):
 # as script in this origin - see shopdb/utils/uploads.py.
 from shopdb.utils.uploads import send_upload, harden_upload_response
 
+# Caller IP for rate limiting and audit. On the surface so a plugin does not
+# reimplement it: the obvious hand-rolled version reads X-Forwarded-For, which
+# the caller supplies, so a rate limit keyed on it can be rotated away to
+# nothing. See shopdb/utils/clientip.py.
+from shopdb.utils.clientip import client_ip, strip_port
+
 # Import-mode helpers: preserve legacy timestamps during a bulk data import
 from shopdb.utils.import_mode import (
     apply_import_timestamps,
@@ -346,6 +352,8 @@ __all__ = [
     # Serving uploads
     'send_upload',
     'harden_upload_response',
+    'client_ip',
+    'strip_port',
     # Import-mode helpers
     'apply_import_timestamps',
     'import_mode_active',