Add transactional email system via SMTP

Introduce internal/email package with SMTP sending, embedded HTML/text
templates, and multipart MIME assembly. Emails use a generic EmailData
struct (recipient name, message, optional button, optional closing) so
new email types can be added without code changes.

Wired into signup (welcome email), team creation, and team member
addition. No-op mailer when SMTP_HOST is not configured.

pkg/config/config.go

@@ -3,6 +3,7 @@ package config
 import (
 	"encoding/hex"
 	"os"
+	"strconv"
 
 	"github.com/joho/godotenv"
 )
@@ -27,6 +28,13 @@ type Config struct {
 	// Channels — encryption for channel secrets (AES-256-GCM).
 	EncryptionKeyHex string   // WRENN_ENCRYPTION_KEY raw hex string (for validation)
 	EncryptionKey    [32]byte // parsed 32-byte key
+
+	// SMTP — transactional email. All fields optional; omitting SMTPHost disables email.
+	SMTPHost      string // SMTP_HOST
+	SMTPPort      int    // SMTP_PORT (default 587)
+	SMTPUsername  string // SMTP_USERNAME
+	SMTPPassword  string // SMTP_PASSWORD
+	SMTPFromEmail string // SMTP_FROM_EMAIL
 }
 
 // Load reads configuration from a .env file (if present) and environment variables.
@@ -50,6 +58,12 @@ func Load() Config {
 		CPPublicURL:             os.Getenv("CP_PUBLIC_URL"),
 
 		EncryptionKeyHex: os.Getenv("WRENN_ENCRYPTION_KEY"),
+
+		SMTPHost:      os.Getenv("SMTP_HOST"),
+		SMTPPort:      envOrDefaultInt("SMTP_PORT", 587),
+		SMTPUsername:  os.Getenv("SMTP_USERNAME"),
+		SMTPPassword:  os.Getenv("SMTP_PASSWORD"),
+		SMTPFromEmail: envOrDefault("SMTP_FROM_EMAIL", "noreply@wrenn.dev"),
 	}
 
 	if cfg.EncryptionKeyHex != "" {
@@ -68,3 +82,15 @@ func envOrDefault(key, def string) string {
 	}
 	return def
 }
+
+func envOrDefaultInt(key string, def int) int {
+	v := os.Getenv(key)
+	if v == "" {
+		return def
+	}
+	n, err := strconv.Atoi(v)
+	if err != nil {
+		return def
+	}
+	return n
+}