Store powder specific gravity and fix coverage math

src/PowderCoating.Core/Entities/InventoryItem.cs

@@ -20,6 +20,7 @@ public class InventoryItem : BaseEntity
     public string? Manufacturer { get; set; }
     public string? ManufacturerPartNumber { get; set; }
     public decimal? CoverageSqFtPerLb { get; set; } // Square feet coverage per pound (default 30)
+    public decimal? SpecificGravity { get; set; } // Powder specific gravity from the technical data sheet
     public decimal? TransferEfficiency { get; set; } // Percentage of powder that sticks (default 65%)
     public decimal? CureTemperatureF { get; set; } // Required cure temperature in °F (recommended for oven scheduling)
     public int? CureTimeMinutes { get; set; } // Required hold time at cure temperature

src/PowderCoating.Core/Entities/PowderCatalogItem.cs

@@ -52,6 +52,9 @@ public class PowderCatalogItem
     /// <summary>Theoretical coverage in sq ft per pound. Typical 80–120.</summary>
     public decimal? CoverageSqFtPerLb { get; set; }
 
+    /// <summary>Specific gravity from the TDS. Used to derive theoretical coverage when needed.</summary>
+    public decimal? SpecificGravity { get; set; }
+
     /// <summary>Powder transfer efficiency percentage. Typical 60–75%.</summary>
     public decimal? TransferEfficiency { get; set; }