Move HAProxy into the host network namespace so it sees the real
client source IP on inbound connections.  With bridge networking +
published ports the source IP is rewritten to the bridge gateway by
the runtime (Docker's userland-proxy, rootless Podman's slirp4netns
or pasta), and the PROXY v2 header HAProxy then sends to mtg and
Caddy carries that useless address.

mtg and Caddy stay on the compose bridge and publish their ports on
host loopback; the host-mode HAProxy dials them at 127.0.0.1.  Caddy's
proxy_protocol allow list is tightened to loopback only.

The 'sysctls: net.ipv4.ip_unprivileged_port_start=80' line is removed
because Docker refuses to apply namespaced sysctls when the netns is
shared with the host.  Rootless Podman users binding the privileged
ports need the equivalent host-side sysctl once; this is documented
in README.md.

Fixes #498.

mtg now also sends PROXY v2 on the fronting dial (introduced in the
previous commit via [domain-fronting].proxy-protocol = true), so the
"Real client IPs" section's sync list must include that fourth piece.
Without it, an operator who disables Caddy's PROXY listener wrapper
without also flipping [domain-fronting].proxy-protocol will leave mtg
sending an unparsed PROXY v2 prefix to Caddy on every fronted probe.

When the secret's domain resolves back to this server (the SNI-router
default), mtg's fallback fronting dial lands on HAProxy, the SNI
matches the secret, HAProxy routes the connection back to mtg -> loop.

Set [domain-fronting].host = "web" in mtg-config.toml so mtg dials
Caddy directly via compose-network DNS, bypassing HAProxy.  Requires
mtg >= 2.4 (#480 added hostname acceptance for the fronting target).

README gains a "Fronting loop" section explaining the cause.

Closes #501.

Before: the SNI hostname had to be edited in haproxy.cfg, plus
DOMAIN had to be set in .env for Caddy.  Two places to keep in sync.

After: DOMAIN is the single source — set it once in .env (or export),
docker-compose forwards it into both haproxy and caddy containers,
and HAProxy interpolates ${DOMAIN} into the SNI ACL at startup.

mtg-config.toml's secret still embeds the domain in its bytes
(generated once with `mtg generate-secret <domain>`), so that one
remains a one-time edit at install — the README is updated to reflect
this.

HAProxy has supported environment-variable substitution in config
strings since 1.6.

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.

Add an ACL that routes /.well-known/acme-challenge/ requests on :80
to Caddy instead of redirecting to HTTPS, so Let's Encrypt certificate
issuance works out of the box.

Also simplify Caddyfile to use Caddy's http_port/https_port directives.

Turnkey deployment: HAProxy on :443 peeks at the TLS SNI and routes
Telegram clients to mtg while forwarding everything else (including DPI
probes) to a real Caddy web server with automatic HTTPS.

This is the setup recommended in BEST_PRACTICES.md, packaged so that
operators can clone and run it with minimal configuration.

Refs: #458