feat: support STARTTLS on plaintext listeners

[iamd3vil]

Summary

Adds STARTTLS support (RFC 3207). Listeners with TLS config now take a mode field:

[[server.listeners]]
addr = "0.0.0.0:587"
[server.listeners.tls]
cert_path = "/etc/ssl/certs/smtp.crt"
key_path = "/etc/ssl/private/smtp.key"
mode = "starttls"   # "implicit" is the default, so existing configs are unchanged

• implicit (default): TLS handshake before any SMTP traffic — existing behavior for port 465.
• starttls: connection starts in plaintext, server advertises 250-STARTTLS, and the session upgrades to TLS when the client issues STARTTLS — for submission on port 587.

What was broken

The STARTTLS path in the smtp crate was dead code (see PRODUCTION_HARDENING.md item 8, now marked addressed):

1. The parser only matched "STARTTLS\r\n", but the read loop strips CRLF before parsing, so real clients always got 500 Syntax error.
2. main.rs never gave the session a TLS acceptor, so STARTTLS was never advertised.
3. The upgrade duplicated the raw fd via unsafe { from_raw_fd }, creating two owners of the same socket (double-close).

Implementation

• New smtp::MaybeTlsStream enum upgrades plain → TLS in place via mem::replace — no unsafe, no recursive session handling. The acceptor travels with the stream, so each listener independently decides whether to offer STARTTLS (different certs per listener keep working).
• STARTTLS is rejected with 502 when unavailable or already on TLS, 503 mid-transaction.
• Security: bytes pipelined after the STARTTLS command are discarded before the handshake (plaintext command injection defense), and the session fully resets afterwards — the client must EHLO and re-authenticate.
• DoS hardening: the TLS handshake is bounded by cmd_timeout on both the STARTTLS upgrade and the implicit-TLS accept, so clients that go silent mid-handshake can't hold max_connections permits indefinitely.
• Fix: QUIT now gets exactly one 221 (previously handle_client wrote a second Bye on clean exit, causing broken-pipe error logs for fast-closing clients).
• Removed the now-dead SmtpServer::with_tls and the smtp crate's rustls/rustls-pemfile deps; example configs and docs updated.

Testing

• cargo test: 153 tests pass; parser tests updated for the corrected STARTTLS contract. Clippy clean on touched code.
• Live end-to-end run against a debug build with plaintext + implicit-TLS + STARTTLS listeners (dev certs, cmd_timeout = "3s"), 10 cases, all passing:
  • plain and implicit TLS flows unchanged; STARTTLS advertise → upgrade → AUTH → send with cert verification
  • 502 on plain listeners and after upgrade; 503 mid-transaction
  • pipelined plaintext after STARTTLS is provably discarded
  • silent clients after STARTTLS (and on the implicit-TLS port) are dropped at the timeout
  • single 221 on QUIT, zero errors in server logs

The e2e checks live in a session-local script; porting them into smtp/tests/ as Rust integration tests is a possible follow-up.