Client address is the right-most untrusted X-Forwarded-For entry (closes #144)

From a peer in TRUSTED_PROXIES, clientIP took the left-most
X-Forwarded-For entry, which the client writes itself behind a proxy
that appends to the header, so a client could choose the address the
rate limit counts and the request log records. It now walks the entries
from the right past trusted proxies and takes the first that is not
one; with only trusted entries, or one that is not an address, it uses
the peer. Every X-Forwarded-For header line is read. X-Real-IP is read
only when there is no X-Forwarded-For entry. README.md Settings says
which entry is taken.

Model: opus-5-5

TODO.md

@@ -21,6 +21,18 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-18: from a trusted proxy, the backend takes the client address from
+  the right-most `X-Forwarded-For` entry that is not in `TRUSTED_PROXIES`
+  ([#144](https://git.eeqj.de/sneak/netwatch/issues/144)). It took the left-most
+  entry, which the client writes itself behind a proxy that appends to the
+  header, so a client could choose the address the rate limit counts and the
+  request log records. With only trusted addresses in the header, or an entry
+  that is not an IP address met on the way, the direct peer is used. `X-Real-IP`
+  is read only without `X-Forwarded-For`, and every `X-Forwarded-For` header
+  line is read, not only the first. The image's nginx, and the
+  `TRUSTED_PROXIES=127.0.0.1/32` it starts the backend with, are unchanged. Unit
+  tests resolve the address from chains ending in an untrusted address, in
+  trusted ones, of only trusted ones, and split over two header lines
 - 2026-10-18: after an interval change, the summary's Reachable slot reads
   `--/--` in white, as at page load, until the second round since the change
   ends ([#140](https://git.eeqj.de/sneak/netwatch/issues/140)). It used to read