feat: tipos y funciones email SMTP en Go (infra)

Tipos: EmailAttachment, EmailMessage, SMTPConfig.
Puras: email_build_html, email_build_text, email_with_attachment, email_template_render.
Impuras: smtp_connect (TLS/STARTTLS/plain), smtp_send (MIME multipart con adjuntos).
Solo stdlib: net/smtp, crypto/tls, text/template, mime/multipart.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/infra/smtp_connect.go

@@ -0,0 +1,81 @@
+package infra
+
+import (
+	"crypto/tls"
+	"fmt"
+	"net"
+	"net/smtp"
+)
+
+// SMTPConnect establishes an authenticated SMTP connection using the given config.
+// TLSMode controls encryption:
+//   - "tls"      → direct TLS (typical port 465)
+//   - "starttls" → plain connection upgraded with STARTTLS (typical port 587)
+//   - ""         → no encryption (typical port 25)
+//
+// Returns an *smtp.Client ready to use with SMTPSend.
+// The caller is responsible for calling client.Quit() when done.
+func SMTPConnect(cfg SMTPConfig) (*smtp.Client, error) {
+	addr := fmt.Sprintf("%s:%d", cfg.Host, cfg.Port)
+
+	switch cfg.TLSMode {
+	case "tls":
+		tlsCfg := &tls.Config{ServerName: cfg.Host}
+		conn, err := tls.Dial("tcp", addr, tlsCfg)
+		if err != nil {
+			return nil, fmt.Errorf("smtp_connect: tls dial %s: %w", addr, err)
+		}
+		client, err := smtp.NewClient(conn, cfg.Host)
+		if err != nil {
+			conn.Close()
+			return nil, fmt.Errorf("smtp_connect: new client: %w", err)
+		}
+		if cfg.Username != "" {
+			auth := smtp.PlainAuth("", cfg.Username, cfg.Password, cfg.Host)
+			if err := client.Auth(auth); err != nil {
+				client.Close()
+				return nil, fmt.Errorf("smtp_connect: auth: %w", err)
+			}
+		}
+		return client, nil
+
+	case "starttls":
+		conn, err := net.Dial("tcp", addr)
+		if err != nil {
+			return nil, fmt.Errorf("smtp_connect: dial %s: %w", addr, err)
+		}
+		client, err := smtp.NewClient(conn, cfg.Host)
+		if err != nil {
+			conn.Close()
+			return nil, fmt.Errorf("smtp_connect: new client: %w", err)
+		}
+		tlsCfg := &tls.Config{ServerName: cfg.Host}
+		if err := client.StartTLS(tlsCfg); err != nil {
+			client.Close()
+			return nil, fmt.Errorf("smtp_connect: starttls: %w", err)
+		}
+		if cfg.Username != "" {
+			auth := smtp.PlainAuth("", cfg.Username, cfg.Password, cfg.Host)
+			if err := client.Auth(auth); err != nil {
+				client.Close()
+				return nil, fmt.Errorf("smtp_connect: auth: %w", err)
+			}
+		}
+		return client, nil
+
+	default:
+		// no encryption
+		client, err := smtp.Dial(addr)
+		if err != nil {
+			return nil, fmt.Errorf("smtp_connect: dial %s: %w", addr, err)
+		}
+		if cfg.Username != "" {
+			auth := smtp.PlainAuth("", cfg.Username, cfg.Password, cfg.Host)
+			if err := client.Auth(auth); err != nil {
+				client.Close()
+				return nil, fmt.Errorf("smtp_connect: auth: %w", err)
+			}
+		}
+		return client, nil
+	}
+}