refactor: Replace interface{} with any alias and optimize string formatting with fmt.Appendf

- Replace map[string]interface{} with map[string]any in GraphQL, metrics, payment gateway, and webhook handlers
- Replace []byte(fmt.Sprintf()) with fmt.Appendf(nil, ...) for cursor encoding in GraphQL resolvers and tests
- Replace strings.HasPrefix/TrimPrefix with strings.CutPrefix for webhook signature parsing
- Replace stripe.Int64/Bool with new() constructor for Stripe payment parameters
- Rename Error

apps/api-server/internal/handlers/subscriber.go

@@ -264,7 +264,8 @@ func (h *SubscriberHandler) TerminateSubscriber(c *gin.Context) {
 }
 
 // ErrorResponse represents an API error response
-type ErrorResponse struct {
+type SubscriberErrorResponse struct {
 	Error   string `json:"error"`
+	Code    string `json:"code"`
 	Details string `json:"details,omitempty"`
 }