`MTG_SECRET=<placeholder> envsubst < ...` was shell-broken on literal
copy-paste — bash parses `<placeholder>` as redirection from a
non-existent file. Two-line `export MTG_SECRET=...` + plain envsubst
form removes the ambiguity. Applies to README, docker-compose.yml,
and the .example header.

Track `mtg-config.toml.example` with `secret = "${MTG_SECRET}"`; the
rendered `mtg-config.toml` and local `.env` are gitignored, so the
secret never lands in a tracked file.

Quick start switches from "paste the secret into mtg-config.toml" to
either `envsubst < mtg-config.toml.example > mtg-config.toml` or
`cp` + hand-edit `${MTG_SECRET}` for users without envsubst.

After #502 made DOMAIN env-driven, the secret was the last hand-edit
of a tracked file in the example. Follow-up to #506.

OpenWrt firewall zones are bound to interface names. With bare podman
you can pin the static podman0 bridge into a zone, but podman-compose
creates a project-scoped network and netavark spawns a fresh bridge
(podman1, podman2, ...) per project — with no firewall rules — so
containers lose outbound access.

Mark the default network as external/name=podman to attach to the
router-managed podman0 instead.

Background: #513.

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