Add NoOp email service, /docs shortcut and fix streak timezone bug

Replace nil email service pattern with NoOpEmailService (Null Object)
to eliminate nil pointer panics across all handlers.

Add /docs route as a shortcut to Swagger UI.

Fix streak calculation returning 0 when server timezone differs from
UTC — CreatedAt was not converted to UTC before date extraction.

internal/application/commands/register_user.go

@@ -73,7 +73,7 @@ func (h *RegisterUserHandler) Handle(ctx context.Context, cmd RegisterUserComman
 	user := entities.NewUser(cmd.Email, hashedPassword)
 
 	emailVerificationRequired := false
-	if h.emailService != nil {
+	if h.emailService.IsEnabled() {
 		token, err := h.generateVerificationToken()
 		if err != nil {
 			return nil, fmt.Errorf("failed to generate verification token: %w", err)