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

Cargo.toml

@@ -20,6 +20,7 @@ hex = "0.4.3"
 hmac = "0.13.0"
 image = "0.25.10"
 jiff = { version = "0.2.35", features = ["serde"] }
+lettre = { version = "0.11", default-features = false, features = ["builder", "smtp-transport", "tokio1", "tokio1-rustls-tls", "hostname"] }
 libc = "0.2.189"
 password-auth = "1.0.0"
 rand = "0.10.2"