Store powder specific gravity and fix coverage math

src/PowderCoating.Web/Controllers/InventoryController.cs

@@ -747,6 +747,7 @@ public class InventoryController : Controller
             if (match.ColorFamilies      != null) result.ColorFamilies      = match.ColorFamilies;
             if (match.RequiresClearCoat  != null) result.RequiresClearCoat  = match.RequiresClearCoat;
             if (match.CoverageSqFtPerLb  != null) result.CoverageSqFtPerLb  = match.CoverageSqFtPerLb;
+            if (match.SpecificGravity    != null) result.SpecificGravity    = match.SpecificGravity;
             result.TransferEfficiency ??= GetEffectiveTransferEfficiency(match.TransferEfficiency);
             // URL / price fields: fill gaps only — AI may have found something better
             result.ImageUrl    ??= match.ImageUrl;
@@ -777,6 +778,7 @@ public class InventoryController : Controller
                 ColorFamilies      = result.ColorFamilies,
                 RequiresClearCoat  = result.RequiresClearCoat,
                 CoverageSqFtPerLb  = result.CoverageSqFtPerLb,
+                SpecificGravity    = result.SpecificGravity,
                 TransferEfficiency = GetEffectiveTransferEfficiency(result.TransferEfficiency),
                 ImageUrl           = result.ImageUrl,
                 ProductUrl         = result.SpecPageUrl,
@@ -875,6 +877,7 @@ public class InventoryController : Controller
                         aiResult.CureTimeMinutes        ??= full.CureTimeMinutes;
                         aiResult.RequiresClearCoat      ??= full.RequiresClearCoat;
                         aiResult.CoverageSqFtPerLb      ??= full.CoverageSqFtPerLb;
+                        aiResult.SpecificGravity        ??= full.SpecificGravity;
                         aiResult.TransferEfficiency     ??= GetEffectiveTransferEfficiency(full.TransferEfficiency);
                         aiResult.ManufacturerPartNumber ??= full.ManufacturerPartNumber;
                         aiResult.ColorName              ??= full.ColorName;
@@ -954,6 +957,7 @@ public class InventoryController : Controller
             colorFamilies          = aiResult.ColorFamilies,
             requiresClearCoat      = aiResult.RequiresClearCoat,
             coverageSqFtPerLb      = aiResult.CoverageSqFtPerLb,
+            specificGravity        = aiResult.SpecificGravity,
             transferEfficiency     = aiResult.TransferEfficiency ?? DefaultTransferEfficiency,
             unitPrice              = aiResult.UnitCostPerLb ?? 0m,
             imageUrl               = aiResult.ImageUrl,
@@ -1106,6 +1110,7 @@ public class InventoryController : Controller
                 colorFamilies = p.ColorFamilies,
                 requiresClearCoat = p.RequiresClearCoat,
                 coverageSqFtPerLb = p.CoverageSqFtPerLb,
+                specificGravity = p.SpecificGravity,
                 transferEfficiency = GetEffectiveTransferEfficiency(p.TransferEfficiency)
             })
             .ToList();