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.

Might be needed for local chained proxy setup.

Fix SELinux-related permission denied error for containerized apps
reading configs exposed via volumes.
Also make it possible to use port 80 in the fronted.

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