docs: document trusted_proxies and record in TODO.md

Describe the trusted_proxies key in README.md and config.example.yml,
including the rightmost-non-proxy selection rule and the safe default of
trusting no one. Record the unit in TODO.md.

Model: opus-4-8

README.md

@@ -126,6 +126,15 @@ Configured via YAML file (`--config`). Key settings:
   added to the always-enforced built-in ranges (loopback, private,
   link-local, CGNAT, benchmark, NAT64, and the like); an invalid CIDR
   aborts startup
+- `trusted_proxies` — list of CIDR ranges of the reverse proxies in front
+  of pixa. `X-Forwarded-For` is believed only when the direct peer falls
+  inside one of these ranges; the logged and login-recorded client
+  address is then the rightmost forwarded entry that is not itself a
+  trusted proxy. Otherwise the direct peer address is used and the header
+  is ignored, so a client connecting directly cannot spoof its address.
+  Omitted or empty trusts no one; an invalid CIDR aborts startup. Set
+  this to your proxy's address range when deploying behind a reverse
+  proxy
 - `upstream_fetch_timeout` — timeout for origin requests
 - `upstream_max_response_size` — max origin response size
 - `downstream_timeout` — client response timeout

TODO.md

@@ -30,6 +30,17 @@ exhaustion
 
 # Completed Steps
 
+- 2026-09-21 trusted-proxy client IP resolution (closes #94): a
+  `trusted_proxies` config key taking a list of CIDRs, parsed by the same
+  `net/netip` list parser as `blocked_networks` (an invalid entry aborts
+  startup naming the key and value; omitted or empty trusts no one); a new
+  `internal/clientip` package resolves the client address by honoring
+  `X-Forwarded-For` only when the direct peer is a trusted proxy, walking
+  the chain right-to-left to the rightmost non-proxy entry, so a client
+  connecting directly cannot spoof its address; the resolved address is
+  stored in the request context by a new middleware and used by the
+  request-logging middleware and the login-attempt logs in place of the
+  raw peer address; documented in `README.md` and `config.example.yml`.
 - 2026-09-21 blocked networks configuration extending SSRF protection: a
   `blocked_networks` config key taking a list of CIDRs (parsed with
   `net/netip`, an invalid entry aborts startup naming the key and value),

config.example.yml

@@ -31,6 +31,18 @@ allowlist_hosts:
 #   - 100.64.0.0/10
 #   - 2001:db8::/32
 
+# CIDR ranges of the reverse proxies in front of pixa. X-Forwarded-For
+# is believed only when the direct peer is inside one of these ranges;
+# the client address in the access log and login records is then the
+# rightmost forwarded entry that is not itself a trusted proxy. A client
+# connecting directly (peer outside these ranges) cannot spoof its
+# address: the header is ignored and the peer address is used. Omitted or
+# empty trusts no one; an invalid CIDR aborts startup. Set this when
+# deploying behind a proxy.
+# trusted_proxies:
+#   - 10.0.0.0/8
+#   - 2001:db8::/32
+
 # Allow HTTP upstream (only for testing, always use HTTPS in production)
 allow_http: false