Add optional email verification and registration control

Implemented email service infrastructure with SMTP support and optional email verification for self-hosted deployments. Registration flow now supports open/closed modes and hardcoded Apocapoc branding.

Key features:
- Email service with SMTP and template rendering
- Optional email verification (auto-verified without SMTP config)
- Registration modes: open/closed for access control
- Hardcoded Apocapoc branding (AppName, AppURL, DefaultFrom)
- Separate registration and login flows (registration no longer returns tokens)

internal/domain/services/email_service.go

@@ -0,0 +1,12 @@
+package services
+
+type EmailMessage struct {
+	To      string
+	Subject string
+	Body    string
+	IsHTML  bool
+}
+
+type EmailService interface {
+	Send(message EmailMessage) error
+}