fix: security tests with mock email sender and rate limit isolation

- Add EmailSender interface to allow mocking in tests
- Add IsInitialized() method to template.Manager for nil-safe checks
- Update contact handler to use interface and safe initialization checks
- Add mockEmailSender in security tests to avoid SMTP connection attempts
- Use unique IPs per test case to avoid rate limiting interference

internal/templates/template.go

@@ -17,6 +17,12 @@ type Manager struct {
 	mu        sync.RWMutex
 }
 
+// IsInitialized returns true if the template manager has been properly initialized
+// with a config. Empty Manager structs (e.g., in tests) will return false.
+func (m *Manager) IsInitialized() bool {
+	return m != nil && m.config != nil
+}
+
 // NewManager creates a new template manager
 func NewManager(cfg *config.TemplateConfig) (*Manager, error) {
 	m := &Manager{