nginx: trust X-Forwarded-For only from TRUSTED_PROXIES (closes #64)

nginx trusted X-Forwarded-For from every RFC1918 address, so a client
reaching it from one could write a new address on each request and
get a fresh rate-limit allowance. The container's TRUSTED_PROXIES now
names the reverse proxies nginx trusts, none by default.
bin/entrypoint.sh refuses a value with a character no IP address or
CIDR has, and writes one set_real_ip_from line per entry into
/etc/nginx/trusted-proxies.conf, which nginx.conf includes. The
backend is started with TRUSTED_PROXIES=127.0.0.1/32, since nginx is
its only client. The viewport test mounts an empty file there.

Model: opus-5-5

TODO.md

@@ -23,6 +23,14 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-09-29: nginx takes the client address from `X-Forwarded-For` only on
+  requests from the reverse proxies named in the container's `TRUSTED_PROXIES`
+  (issue #64), and by default from none, where it trusted every RFC1918 address
+  before, so a client could write a new address on each request and escape the
+  rate limit. `bin/entrypoint.sh` writes one `set_real_ip_from` line per entry
+  into `/etc/nginx/trusted-proxies.conf`, which `nginx.conf` includes, refusing
+  a value with a character no IP address or CIDR has; it starts the backend with
+  `TRUSTED_PROXIES=127.0.0.1/32`, since nginx is its only client
 - 2026-09-29: ready to run under upaas (issue #59): the image has a
   `HEALTHCHECK` that requests `/.well-known/healthcheck` through nginx on the
   port from `PORT`. The backend no longer reads a bad `PORT` as 0 or a bad