fix: shortlink generation logic update/create

internal/api/models/shortlink.go

@@ -27,6 +27,7 @@ type GenerateShortLinkResponse struct {
 	ShortToken string `json:"short_token"`
 	FullURL    string `json:"full_url"`
 	ExpiresAt  string `json:"expires_at"`
+	IsExisting bool   `json:"is_existing"` // Indicates if this is an existing link that was reused
 }
 
 // ShortLinkAuthRequest represents the shortlink authentication request