fix(smtp): proper SMTP dialogue, encoded mails, optional skip-verify

- Explicit SMTP flow (dial -> EHLO -> STARTTLS -> AUTH -> MAIL/RCPT/DATA)
  with errors naming the refusing step (also logged as smtp_step)
- EHLO with the sender's domain instead of "localhost"
- Envelope gets the bare address; "Name <addr>" stays in the From header
  (was rejected as invalid MAIL FROM)
- Date and Message-ID headers, RFC 2047 subject, quoted-printable body
- SMTP_SKIP_VERIFY / admin switch for self-signed certificates;
  SMTP checkboxes are switches now
- Tests against a fake SMTP server (STARTTLS, 451, display names)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/config/config.go

@@ -25,6 +25,8 @@ type Config struct {
 	SMTPPass string
 	SMTPFrom string
 	SMTPTLS  bool
+	// SMTPSkipVerify accepts self-signed or mismatching certificates (internal mail servers)
+	SMTPSkipVerify bool
 }
 
 func Load() *Config {
@@ -60,6 +62,7 @@ func Load() *Config {
 	smtpPass := getEnv("SMTP_PASS", "")
 	smtpFrom := getEnv("SMTP_FROM", "")
 	smtpTLS := strings.ToLower(getEnv("SMTP_TLS", "true")) != "false"
+	smtpSkipVerify := strings.ToLower(getEnv("SMTP_SKIP_VERIFY", "false")) == "true"
 
 	return &Config{
 		Port:              port,
@@ -77,6 +80,7 @@ func Load() *Config {
 		SMTPPass:          smtpPass,
 		SMTPFrom:          smtpFrom,
 		SMTPTLS:           smtpTLS,
+		SMTPSkipVerify:    smtpSkipVerify,
 	}
 }