feat: replace m CLI email with direct SMTP over TLS

The m CLI isn't available in Docker containers. Replace exec.Command("m", "mail", "send")
with direct SMTP using crypto/tls + net/smtp (implicit TLS on port 465).

Env vars: SMTP_HOST, SMTP_PORT, SMTP_USER, SMTP_PASS, MAIL_FROM
Gracefully skips sending if SMTP is not configured.

Note: mgmt@msbls.de rejected by Hostinger as not owned by mail@msbls.de.
Default from address set to mail@msbls.de until alias is created.

backend/internal/config/config.go

@@ -14,6 +14,13 @@ type Config struct {
 	SupabaseJWTSecret string
 	AnthropicAPIKey   string
 	FrontendOrigin    string
+
+	// SMTP settings (optional — email sending disabled if SMTPHost is empty)
+	SMTPHost string
+	SMTPPort string
+	SMTPUser string
+	SMTPPass string
+	MailFrom string
 }
 
 func Load() (*Config, error) {
@@ -26,6 +33,12 @@ func Load() (*Config, error) {
 		SupabaseJWTSecret: os.Getenv("SUPABASE_JWT_SECRET"),
 		AnthropicAPIKey:  os.Getenv("ANTHROPIC_API_KEY"),
 		FrontendOrigin:   getEnv("FRONTEND_ORIGIN", "https://kanzlai.msbls.de"),
+
+		SMTPHost: os.Getenv("SMTP_HOST"),
+		SMTPPort: getEnv("SMTP_PORT", "465"),
+		SMTPUser: os.Getenv("SMTP_USER"),
+		SMTPPass: os.Getenv("SMTP_PASS"),
+		MailFrom: getEnv("MAIL_FROM", "mail@msbls.de"),
 	}
 
 	if cfg.DatabaseURL == "" {