Add SMTP mail, access-request notifications, and a shared request throttle

A new [mail] section configures an SMTP relay (SES-style STARTTLS or
implicit TLS; the password is environment-only via
DAILY_EPUB_MAIL__SMTP_PASS) and src/mail.rs wraps lettre in a small
plain-text Mailer built once at server start. Each stored access request
now emails mail.notify_to in a spawned task with the address, reason,
time, and a link to /dashboard/users. POST /request-access shares the
login endpoint's per-IP limiter (server.login_attempts per window).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QVPagF6jfDv78CC5Jv2wp4

src/mail.rs

@@ -0,0 +1,155 @@
+//! Minimal plain-text SMTP delivery.
+
+#[cfg(test)]
+use std::sync::Arc;
+
+use anyhow::Context as _;
+use lettre::message::{Mailbox, header::ContentType};
+use lettre::transport::smtp::authentication::Credentials;
+use lettre::{AsyncSmtpTransport, AsyncTransport as _, Tokio1Executor};
+
+use crate::config::MailConfig;
+
+/// A plain-text outbound message.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Message {
+    /// Recipient mailbox, either an address or `Name <address>`.
+    pub to: String,
+    /// Message subject.
+    pub subject: String,
+    /// Plain-text message body.
+    pub body: String,
+}
+
+#[derive(Clone)]
+enum Transport {
+    Smtp(AsyncSmtpTransport<Tokio1Executor>),
+    #[cfg(test)]
+    Recording(Arc<std::sync::Mutex<Vec<Message>>>),
+}
+
+/// A reusable SMTP sender built from the startup configuration.
+#[derive(Clone)]
+pub struct Mailer {
+    from: Mailbox,
+    transport: Transport,
+}
+
+impl Mailer {
+    /// Build an SMTP sender, or return `None` when mail is not fully active.
+    pub fn from_config(config: &MailConfig) -> anyhow::Result<Option<Self>> {
+        if !config.is_active() {
+            return Ok(None);
+        }
+
+        let from = parse_mailbox(&config.from, "mail.from")?;
+        let user = config
+            .smtp_user
+            .as_deref()
+            .expect("active mail config has an SMTP username");
+        let pass = config
+            .smtp_pass
+            .as_deref()
+            .expect("active mail config has an SMTP password");
+        let builder = if config.smtp_starttls {
+            AsyncSmtpTransport::<Tokio1Executor>::starttls_relay(&config.smtp_host)
+        } else {
+            AsyncSmtpTransport::<Tokio1Executor>::relay(&config.smtp_host)
+        }
+        .with_context(|| format!("invalid SMTP host {:?}", config.smtp_host))?;
+        let transport = builder
+            .port(config.smtp_port)
+            .credentials(Credentials::new(user.to_string(), pass.to_string()))
+            .build();
+
+        Ok(Some(Self {
+            from,
+            transport: Transport::Smtp(transport),
+        }))
+    }
+
+    /// Send one plain-text message.
+    pub async fn send(&self, message: Message) -> anyhow::Result<()> {
+        #[cfg(test)]
+        if let Transport::Recording(messages) = &self.transport {
+            messages
+                .lock()
+                .unwrap_or_else(|poisoned| poisoned.into_inner())
+                .push(message);
+            return Ok(());
+        }
+
+        let to = parse_mailbox(&message.to, "message recipient")?;
+        let email = lettre::Message::builder()
+            .from(self.from.clone())
+            .to(to)
+            .subject(message.subject)
+            .header(ContentType::TEXT_PLAIN)
+            .body(message.body)
+            .context("building plain-text email")?;
+        match &self.transport {
+            Transport::Smtp(transport) => {
+                transport
+                    .send(email)
+                    .await
+                    .context("sending email through SMTP")?;
+            }
+            #[cfg(test)]
+            Transport::Recording(_) => unreachable!("recording transport returned above"),
+        }
+        Ok(())
+    }
+
+    #[cfg(test)]
+    pub(crate) fn recording() -> (Self, Arc<std::sync::Mutex<Vec<Message>>>) {
+        let messages = Arc::new(std::sync::Mutex::new(Vec::new()));
+        (
+            Self {
+                from: "daily@example.com".parse().expect("valid test mailbox"),
+                transport: Transport::Recording(Arc::clone(&messages)),
+            },
+            messages,
+        )
+    }
+}
+
+fn parse_mailbox(value: &str, field: &str) -> anyhow::Result<Mailbox> {
+    value
+        .parse::<Mailbox>()
+        .with_context(|| format!("{field} is not a valid email mailbox"))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parses_named_and_bare_mailboxes() {
+        assert!(parse_mailbox("The Daily EPUB <daily@example.com>", "from").is_ok());
+        assert!(parse_mailbox("operator@example.com", "to").is_ok());
+        assert!(parse_mailbox("not an address", "to").is_err());
+    }
+
+    #[test]
+    fn inactive_config_builds_no_mailer() {
+        assert!(
+            Mailer::from_config(&MailConfig::default())
+                .unwrap()
+                .is_none()
+        );
+
+        let mut config = MailConfig {
+            enabled: true,
+            smtp_host: "smtp.example.com".into(),
+            from: "daily@example.com".into(),
+            smtp_user: Some("user".into()),
+            smtp_pass: Some("  ".into()),
+            ..MailConfig::default()
+        };
+        assert!(Mailer::from_config(&config).unwrap().is_none());
+
+        config.smtp_pass = Some("secret".into());
+        config.from = "not an address".into();
+        assert!(Mailer::from_config(&config).is_err());
+    }
+}