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>

deploy/nginx.conf.template

@@ -27,7 +27,9 @@ server {
         proxy_set_header Host $host;
         proxy_set_header X-Forwarded-Host $host;
         proxy_set_header X-Forwarded-Proto https;
-        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        # Overwrite, never append: $proxy_add_x_forwarded_for keeps any header the
+        # client sent, and the leftmost value would then be attacker-controlled.
+        proxy_set_header X-Forwarded-For $remote_addr;
         proxy_connect_timeout 5s;
         proxy_read_timeout 30s;
         client_max_body_size 2m;