fix(mail): add support for BCC in email sending

server/internal/config/config.go

@@ -53,6 +53,7 @@ type EmailConfig struct {
 	SMTPPass string `yaml:"smtp_pass"`
 	From     string `yaml:"from"`
 	To       string `yaml:"to"`
+	Bcc      string `yaml:"bcc"` // optional; comma-separated list of extra blind-copy recipients
 }
 
 type UploadConfig struct {