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_test.go

@@ -0,0 +1,97 @@
+package email
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestNewTemplateRenderer(t *testing.T) {
+	renderer := NewTemplateRenderer("Test App", "https://example.com", "support@example.com")
+
+	if renderer == nil {
+		t.Fatal("Expected renderer to be created")
+	}
+
+	if renderer.appName != "Test App" {
+		t.Errorf("Expected app name 'Test App', got '%s'", renderer.appName)
+	}
+}
+
+func TestTemplateRenderer_Render(t *testing.T) {
+	renderer := NewTemplateRenderer("Test App", "https://example.com", "support@example.com")
+
+	template := `Hello {{.Data.Name}}, welcome to {{.AppName}}!`
+	data := map[string]interface{}{
+		"Name": "John",
+	}
+
+	result, err := renderer.Render(template, data)
+	if err != nil {
+		t.Fatalf("Failed to render template: %v", err)
+	}
+
+	expected := "Hello John, welcome to Test App!"
+	if result != expected {
+		t.Errorf("Expected '%s', got '%s'", expected, result)
+	}
+}
+
+func TestTemplateRenderer_RenderWithAllVariables(t *testing.T) {
+	renderer := NewTemplateRenderer("My App", "https://myapp.com", "help@myapp.com")
+
+	template := `
+App: {{.AppName}}
+URL: {{.AppURL}}
+Support: {{.SupportEmail}}
+User: {{.Data.User}}
+`
+	data := map[string]interface{}{
+		"User": "Alice",
+	}
+
+	result, err := renderer.Render(template, data)
+	if err != nil {
+		t.Fatalf("Failed to render template: %v", err)
+	}
+
+	if !strings.Contains(result, "My App") {
+		t.Error("Expected result to contain app name")
+	}
+	if !strings.Contains(result, "https://myapp.com") {
+		t.Error("Expected result to contain app URL")
+	}
+	if !strings.Contains(result, "help@myapp.com") {
+		t.Error("Expected result to contain support email")
+	}
+	if !strings.Contains(result, "Alice") {
+		t.Error("Expected result to contain user name")
+	}
+}
+
+func TestTemplateRenderer_RenderInvalidTemplate(t *testing.T) {
+	renderer := NewTemplateRenderer("Test App", "https://example.com", "support@example.com")
+
+	template := `{{.Data.Invalid}}`
+	data := map[string]interface{}{}
+
+	result, err := renderer.Render(template, data)
+	if err != nil {
+		t.Fatalf("Template should render even with missing data: %v", err)
+	}
+
+	if result != "<no value>" {
+		t.Logf("Got result: %s", result)
+	}
+}
+
+func TestTemplateRenderer_RenderSyntaxError(t *testing.T) {
+	renderer := NewTemplateRenderer("Test App", "https://example.com", "support@example.com")
+
+	template := `{{.Data.Name`
+	data := map[string]interface{}{}
+
+	_, err := renderer.Render(template, data)
+	if err == nil {
+		t.Error("Expected error for invalid template syntax")
+	}
+}