feat: implement B.3 — fatal SELFPOST_HOSTNAME check in entrypoint.sh

Unset or malformed hostname makes the panel and Postfix diverge on SASL
realm silently (auth breaks for every application) and breaks HELO/PTR
matching (spam), so entrypoint.sh now exits before postfix-config.sh /
supervisord with an explanatory error, plus a syntax check rejecting
missing dots, schemes, ports, and whitespace.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -37,6 +37,11 @@ mv .env.example .env   # then edit SELFPOST_HOSTNAME etc.
 docker compose up -d
 ```
 
+`SELFPOST_HOSTNAME` is required — the container exits immediately with an
+explanatory error if it's unset, since it doubles as the Postfix HELO name,
+the SASL realm, and must match both the PTR record and the certificate
+CN/SAN.
+
 This starts SelfPost alone; it assumes Apache is already installed on the host
 as the reverse proxy (see below) and expects certificates at `./certs`. The
 first log line (`docker compose logs -f`) prints the one-time setup link —