Log the client address next to the peer address (closes #270)

The access log, the rate-limit rejection lines, the CSRF warning and
the receiver's "webhook request received" line now carry clientIP,
the address the rate limiters key on, next to remoteIP, the
connecting peer. Logging works it out once per request from the same
code the rate limiters use and stores it on the request context for
the other lines. The CSRF and receiver lines name the peer as
remoteIP instead of remote_addr. The README documents the field and
that it is only as trustworthy as TRUSTED_PROXIES.

Model: opus-5-5

internal/middleware/export_test.go

@@ -132,6 +132,12 @@ func (g *LoginGuard) TrackedKeysForTest() (int, int) {
 // passwordChangeRateLimit constant.
 const PasswordChangeRateLimitConst = passwordChangeRateLimit
 
+// ReplayRateLimitConst exposes the replayRateLimit constant.
+const ReplayRateLimitConst = replayRateLimit
+
+// ResubmitRateLimitConst exposes the resubmitRateLimit constant.
+const ResubmitRateLimitConst = resubmitRateLimit
+
 // ReceiverAggregateMultiplierConst exposes the
 // receiverAggregateMultiplier constant.
 const ReceiverAggregateMultiplierConst = receiverAggregateMultiplier