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/infrastructure/email/template_renderer.go

@@ -0,0 +1,49 @@
+package email
+
+import (
+	"bytes"
+	"fmt"
+	"html/template"
+)
+
+type TemplateData struct {
+	AppName      string
+	AppURL       string
+	SupportEmail string
+	Data         map[string]interface{}
+}
+
+type TemplateRenderer struct {
+	appName      string
+	appURL       string
+	supportEmail string
+}
+
+func NewTemplateRenderer(appName, appURL, supportEmail string) *TemplateRenderer {
+	return &TemplateRenderer{
+		appName:      appName,
+		appURL:       appURL,
+		supportEmail: supportEmail,
+	}
+}
+
+func (r *TemplateRenderer) Render(templateContent string, data map[string]interface{}) (string, error) {
+	tmpl, err := template.New("email").Parse(templateContent)
+	if err != nil {
+		return "", fmt.Errorf("failed to parse template: %w", err)
+	}
+
+	templateData := TemplateData{
+		AppName:      r.appName,
+		AppURL:       r.appURL,
+		SupportEmail: r.supportEmail,
+		Data:         data,
+	}
+
+	var buf bytes.Buffer
+	if err := tmpl.Execute(&buf, templateData); err != nil {
+		return "", fmt.Errorf("failed to execute template: %w", err)
+	}
+
+	return buf.String(), nil
+}