Fix estimate data leaking into competitor/warehouse pricelists

The else branch in CreateForSourceWithProgress was reading from
qt_lot_metadata (estimate snapshot) for any source when no items
were provided. This caused estimate prices, coefficients, manual
prices and meta_prices to be copied verbatim into competitor/warehouse
pricelists when called with empty item list.

Fix: else branch is now guarded to source=="estimate" only.
Any other source with no items returns an explicit error instead
of silently falling back to estimate data.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/services/pricelist/service.go

@@ -248,8 +248,8 @@ func (s *Service) CreateForSourceWithProgress(createdBy, source string, sourceIt
 				PricePeriodDays: srcItem.PricePeriodDays,
 			})
 		}
-	} else {
-		// Default snapshot source for estimate and backward compatibility.
+	} else if source == string(models.PricelistSourceEstimate) {
+		// Estimate snapshot: read current prices from qt_lot_metadata.
 		type LotMetadataWithCategory struct {
 			models.LotMetadata
 			LotCategory string
@@ -315,6 +315,10 @@ func (s *Service) CreateForSourceWithProgress(createdBy, source string, sourceIt
 				MetaPrices:       m.MetaPrices,
 			})
 		}
+	} else {
+		// Non-estimate source with no items provided — refuse to fall back to estimate snapshot.
+		_ = s.repo.Delete(pricelist.ID)
+		return nil, fmt.Errorf("no items provided for source %q: explicit item list required", source)
 	}
 
 	if len(items) == 0 {