Pass real client IPs through with PROXY protocol v2

Without this, mtg and Caddy see HAProxy's container IP for every
connection, which breaks meaningful logging, abuse handling, and any
IP-based blocklist logic.  HAProxy sends a PROXY protocol v2 header on
its TCP backends; mtg enables proxy-protocol-listener, and Caddy wraps
:8443 with a proxy_protocol listener before tls.

The :80 path (ACME HTTP-01 passthrough) is unchanged — client IP there
is not useful and HAProxy's http mode already adds X-Forwarded-For if
anyone wants it.

Requested in https://github.com/9seconds/mtg/pull/462 review.

contrib/sni-router/Caddyfile

 	# ACME HTTP-01 challenges arrive on :80 via HAProxy's acl passthrough.
 	http_port 80
 	https_port 8443
+
+	# HAProxy forwards connections to :8443 with a PROXY protocol v2
+	# header (see haproxy.cfg `send-proxy-v2`).  The proxy_protocol
+	# listener wrapper strips the header and exposes the real client IP
+	# to Caddy's access log.  The `tls` wrapper must follow so that TLS
+	# is terminated on the unwrapped connection.
+	#
+	# `allow` lists the networks permitted to send PROXY headers.  These
+	# ranges cover docker compose's default bridge networks; tighten
+	# them if you pin a specific subnet in docker-compose.yml.
+	servers :8443 {
+		listener_wrappers {
+			proxy_protocol {
+				timeout 5s
+				allow 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16
+			}
+			tls
+		}
+	}
 }
 
 {$DOMAIN} {

contrib/sni-router/README.md

 docker compose exec mtg mtg access /config/config.toml
 ```
 
+## Real client IPs (PROXY protocol)
+
+HAProxy forwards TCP connections to mtg and Caddy with a PROXY protocol
+v2 header so both backends see the real client IP instead of HAProxy's
+container address.  The three pieces must stay in sync:
+
+- `haproxy.cfg` — `send-proxy-v2` on the `mtg` and `web` backend `server` lines
+- `mtg-config.toml` — `proxy-protocol-listener = true`
+- `Caddyfile` — `listener_wrappers { proxy_protocol { ... } tls }` on `:8443`
+
+If you disable one, disable all three, otherwise the backend will fail
+to parse the connection.
+
 ## ACME (Let's Encrypt) notes
 
 HAProxy passes `/.well-known/acme-challenge/` requests on `:80` to

contrib/sni-router/haproxy.cfg

     default_backend web
 
 backend mtg
-    server mtg mtg:3128
+    # send-proxy-v2 prepends a PROXY protocol v2 header so mtg sees the
+    # real client IP instead of HAProxy's.  mtg must have
+    # `proxy-protocol-listener = true` in its config.
+    server mtg mtg:3128 send-proxy-v2
 
 backend web
-    server web web:8443
+    # send-proxy-v2 prepends a PROXY protocol v2 header so Caddy logs the
+    # real client IP instead of HAProxy's.  Caddy must enable the
+    # proxy_protocol listener wrapper on :8443 (see Caddyfile).
+    server web web:8443 send-proxy-v2
 
 backend web_acme
     mode http

contrib/sni-router/mtg-config.toml

 secret = "PASTE_YOUR_SECRET_HERE"
 bind-to = "0.0.0.0:3128"
 
+# HAProxy in front sends PROXY protocol v2 headers so mtg can see the
+# real client IP.  Keep this in sync with haproxy.cfg (`send-proxy-v2`).
+proxy-protocol-listener = true
+
 [defense.anti-replay]
 enabled = true
 max-size = "1mib"