// Package smtp provides a lifecycle-managed SMTP client for sending email
// in Einherjar applications.
//
// # Overview
//
// [New] returns a [Component] that satisfies [lifecycle.Component] lifecycle
// hooks, [observability.Checkable] with degraded priority, and the [Sender]
// interface for dispatching email. When [Config.Host] is empty, [New] returns
// a no-op implementation that logs a warning and silently discards every message —
// email failure must never block a transaction.
//
// # Lifecycle Registration
//
//	client := smtp.New(logger, cfg)
//	launcher.Register(client)
//	health.Register(client)
//
// # Sending
//
// Services depend only on [Sender]. Build a [Message] with a pre-rendered body,
// then call Send:
//
//	msg := smtp.Message{
//	    To:          []string{"alice@example.com"},
//	    Subject:     "Your receipt",
//	    Body:        rendered,     // pre-rendered string from Template.Render
//	    ContentType: "text/html",
//	}
//	if err := mailer.Send(ctx, msg); err != nil {
//	    return err
//	}
//
// # Template Rendering
//
// [ParseFS] wraps stdlib html/template for HTML email rendering. Rendering is
// intentionally separate from Send — callers render to a string and assign it
// to Message.Body. This keeps Send unit-testable without template involvement.
//
//	tmpl, err := smtp.ParseFS(os.DirFS("templates"), "*.html")
//	body, err := tmpl.Render("receipt.html", data)
//	msg := smtp.Message{Body: body, ContentType: "text/html", ...}
//
// # Configuration
//
// All fields are read from environment variables with the EINHERJAR_SMTP_* prefix:
//
//   - EINHERJAR_SMTP_HOST     — SMTP server hostname (empty = no-op mode)
//   - EINHERJAR_SMTP_PORT     — default: 587 (STARTTLS)
//   - EINHERJAR_SMTP_USER     — auth username (optional)
//   - EINHERJAR_SMTP_PASSWORD — auth password (optional)
//   - EINHERJAR_SMTP_FROM     — envelope sender address
package smtp