feat: make inbound SMTP greeting/EHLO hostname configurable

[iamd3vil]

Fixes #7

Problem

The inbound SMTP listener hardcoded localhost in both the 220 connection greeting and the 250- EHLO reply, regardless of deployment. server.helo_hostname only controls the outbound EHLO name (passed to lettre), so there was no way to configure the inbound identity.

Changes

• New optional server.hostname config field — the identity announced in the inbound 220 greeting and EHLO reply. helo_hostname remains outbound-only, since the two identities legitimately diverge (outbound name is tied to the sending IP's reverse DNS; inbound name is whatever the MX points to).
• Default when unset: OS hostname (via the hostname crate, already in the dependency tree through lettre's hostname feature), with localhost as a last resort if the lookup fails.
• SmtpServer in the smtp crate gains a with_hostname() builder; the library default stays localhost, keeping the crate free of config/OS-lookup concerns. Resolution happens in smtp-server/src/main.rs, and the resolved name is logged at startup.
• Updated config.example.toml, config.example.huml, and the docs (configuration page + reference examples).

Verification

• cargo test --workspace: 153 passed, 0 failed.
• Live probe with hostname = "mx.verify.test":

  220 mx.verify.test ESMTP server ready
  250-mx.verify.test
  250-SIZE 26214400
  250 OK
  

• Live probe with the field unset: banner shows the machine's OS hostname.