fix: address audit findings for websearch, email verification, and pricing

- Fix websearch provider failover: proxy error from provider-specific proxy
  now continues to next provider instead of aborting the entire loop
- Fix SMTP failure locking users out: send email first, then write cache
  and increment rate counter
- Fix notify email cache key case sensitivity: normalize to lowercase
- Add OriginalPrice validation to validatePlanPatch and validatePlanRequired
- Add empty scope validation for channel pricing rules (group_ids/account_ids)
- Add platform color to account search dropdown in channel pricing rules

backend/internal/service/payment_config_plans.go

@@ -12,7 +12,7 @@ import (
 )
 
 // validatePlanRequired checks that all required fields for a plan are provided.
-func validatePlanRequired(name string, groupID int64, price float64, validityDays int, validityUnit string) error {
+func validatePlanRequired(name string, groupID int64, price float64, validityDays int, validityUnit string, originalPrice *float64) error {
 	if strings.TrimSpace(name) == "" {
 		return infraerrors.BadRequest("PLAN_NAME_REQUIRED", "plan name is required")
 	}
@@ -28,6 +28,9 @@ func validatePlanRequired(name string, groupID int64, price float64, validityDay
 	if strings.TrimSpace(validityUnit) == "" {
 		return infraerrors.BadRequest("PLAN_VALIDITY_UNIT_REQUIRED", "validity unit is required")
 	}
+	if originalPrice != nil && *originalPrice < 0 {
+		return infraerrors.BadRequest("PLAN_ORIGINAL_PRICE_INVALID", "original price must be >= 0")
+	}
 	return nil
 }
 
@@ -48,6 +51,9 @@ func validatePlanPatch(req UpdatePlanRequest) error {
 	if req.ValidityUnit != nil && strings.TrimSpace(*req.ValidityUnit) == "" {
 		return infraerrors.BadRequest("PLAN_VALIDITY_UNIT_REQUIRED", "validity unit is required")
 	}
+	if req.OriginalPrice != nil && *req.OriginalPrice < 0 {
+		return infraerrors.BadRequest("PLAN_ORIGINAL_PRICE_INVALID", "original price must be >= 0")
+	}
 	return nil
 }
 
@@ -115,7 +121,7 @@ func (s *PaymentConfigService) ListPlansForSale(ctx context.Context) ([]*dbent.S
 }
 
 func (s *PaymentConfigService) CreatePlan(ctx context.Context, req CreatePlanRequest) (*dbent.SubscriptionPlan, error) {
-	if err := validatePlanRequired(req.Name, req.GroupID, req.Price, req.ValidityDays, req.ValidityUnit); err != nil {
+	if err := validatePlanRequired(req.Name, req.GroupID, req.Price, req.ValidityDays, req.ValidityUnit, req.OriginalPrice); err != nil {
 		return nil, err
 	}
 	b := s.entClient.SubscriptionPlan.Create().