In-memory email queue implementing `EmailQueueProtocol`.

A lightweight, single-process queued-sending mode: `queue-email!` enqueues,
`process-queue!` drains one entry through the wrapped sender, with bounded
retry (failed sends are re-queued up to `:max-retries` times). Backed by a
thread-safe `ConcurrentLinkedDeque`.

For distributed queuing across replicas use `jobs-integration` (jobs module)
instead — this adapter's state is per-process and lost on restart.

SMTP email adapter — delegates raw transport to libs/external SmtpProviderAdapter.

This adapter sits at the boundary between the email library's domain model
(Email with :id, :created-at, :attachments, :metadata) and the external
library's transport layer (OutboundEmail). It translates between the two and
delegates all javax.mail work to boundary.external.shell.adapters.smtp.

Responsibility split:
  libs/email  — Email domain model, validation, preparation, job queuing
  libs/external — Raw SMTP transport (javax.mail, TLS/SSL, HTML multipart)

Usage:
  (def sender (create-smtp-sender
                {:host "smtp.gmail.com"
                 :port 587
                 :username "user@gmail.com"
                 :password "app-password"
                 :tls? true}))
  (send-email! sender prepared-email)