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.lock

@@ -902,6 +902,7 @@ dependencies = [
  "hmac",
  "image",
  "jiff",
+ "lettre",
  "libc",
  "password-auth",
  "rand 0.10.2",
@@ -1133,6 +1134,22 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "email-encoding"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "420b9da095f052ea597503e39073b5b3c522f7db933fbac202d91d24492693fd"
+dependencies = [
+ "base64 0.23.1",
+ "memchr",
+]
+
+[[package]]
+name = "email_address"
+version = "0.2.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e079f19b08ca6239f47f8ba8509c11cf3ea30095831f7fed61441475edd8c449"
+
 [[package]]
 name = "encoding_rs"
 version = "0.8.35"
@@ -1700,6 +1717,17 @@ dependencies = [
  "digest 0.11.3",
 ]
 
+[[package]]
+name = "hostname"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "617aaa3557aef3810a6369d0a99fac8a080891b68bd9f9812a1eeda0c0730cbd"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "windows-link",
+]
+
 [[package]]
 name = "html-escape"
 version = "0.2.15"
@@ -2226,6 +2254,34 @@ version = "0.5.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7a79a3332a6609480d7d0c9eab957bca6b455b91bb84e66d19f5ff66294b85b8"
 
+[[package]]
+name = "lettre"
+version = "0.11.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f2c646bd5cc763b1087b15493e29a64be6147ba8f19342004fa52048ee596eae"
+dependencies = [
+ "async-trait",
+ "base64 0.23.1",
+ "email-encoding",
+ "email_address",
+ "fastrand",
+ "futures-io",
+ "futures-util",
+ "hostname",
+ "httpdate",
+ "idna",
+ "mime",
+ "nom",
+ "percent-encoding",
+ "quoted_printable",
+ "rustls",
+ "socket2",
+ "tokio",
+ "tokio-rustls",
+ "url",
+ "webpki-roots",
+]
+
 [[package]]
 name = "libc"
 version = "0.2.189"
@@ -2967,6 +3023,12 @@ dependencies = [
  "proc-macro2",
 ]
 
+[[package]]
+name = "quoted_printable"
+version = "0.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "478e0585659a122aa407eb7e3c0e1fa51b1d8a870038bd29f0cf4a8551eea972"
+
 [[package]]
 name = "r-efi"
 version = "5.3.0"
@@ -3318,6 +3380,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0283386ce02abc0151e1761d08802dfe86c173b0b494af5cbc086574e453da06"
 dependencies = [
  "aws-lc-rs",
+ "log",
  "once_cell",
  "ring",
  "rustls-pki-types",