Add Columbia Coatings catalog integration schema fields

Phase 1a of the Columbia Coatings API integration. Adds the persisted
fields the sync/mapper will need, ahead of the client and sync service:

PowderCatalogItem:
- Category: our product category (e.g. "Powder Additives" for gram-sold
  pigments) derived from vendor taxonomy at import, not stored raw.
- Source: provenance (e.g. "Columbia Coatings API"), kept separate from
  VendorName (= derived manufacturer) so a distributor's right-to-delete
  can purge by feed regardless of manufacturer.
- ChemistryType: resin chemistry (Polyester/TGIC/Epoxy/...), distinct
  from Finish.
- MilThickness: recommended film build as vendor free text.
- CureScheduleText: raw cure schedule verbatim (formats vary widely).
- CureCurvesJson: all parsed cure curves, so alternate low-temp curves
  are preserved for heat-sensitive substrates, not just the primary.
- FormulationChanges: vendor reformulation log; a signal cure specs may
  have changed.

InventoryItem:
- PowderCatalogItemId: loose link to the catalog row (matches the
  QuoteItemCoat pattern) so inventory detail can show manufacturer-level
  status (e.g. discontinued/cannot reorder) and future change flags.
  Nulled, never cascaded, when source catalog data is purged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/PowderCoating.Core/Entities/PowderCatalogItem.cs

@@ -40,9 +40,30 @@ public class PowderCatalogItem
     /// <summary>Cure hold time at cure temperature, in minutes.</summary>
     public int? CureTimeMinutes { get; set; }
 
+    /// <summary>
+    /// Raw cure schedule text exactly as supplied by the vendor — e.g. "10 minutes @ 400°F".
+    /// Preserved verbatim because vendor formats vary wildly and some carry application notes
+    /// that don't reduce to a single temp/time pair (partial cures, clear-coat steps).
+    /// </summary>
+    public string? CureScheduleText { get; set; }
+
+    /// <summary>
+    /// All parsed cure curves as JSON — e.g. [{"tempF":400,"minutes":10},{"tempF":350,"minutes":20}].
+    /// Many powders list alternate lower-temperature curves; these matter for heat-sensitive
+    /// substrates that cannot take the standard 400°F cure, so we keep every curve, not just the
+    /// primary one in <see cref="CureTemperatureF"/>/<see cref="CureTimeMinutes"/>.
+    /// </summary>
+    public string? CureCurvesJson { get; set; }
+
     /// <summary>Finish type — e.g. Gloss, Matte, Satin, Metallic, Texture.</summary>
     public string? Finish { get; set; }
 
+    /// <summary>Resin chemistry — e.g. "Polyester", "TGIC", "Epoxy", "Hybrid". Distinct from <see cref="Finish"/>.</summary>
+    public string? ChemistryType { get; set; }
+
+    /// <summary>Recommended film build (mil thickness) as free text from the vendor — e.g. "2.0-3.0 Mils".</summary>
+    public string? MilThickness { get; set; }
+
     /// <summary>Comma-separated color family tags — e.g. "Blue,Purple".</summary>
     public string? ColorFamilies { get; set; }
 
@@ -60,6 +81,29 @@ public class PowderCatalogItem
 
     // ── Catalog management ────────────────────────────────────────────────
 
+    /// <summary>
+    /// Our internal product category — e.g. "Powder Additives" for pigments/additives that are
+    /// sold by weight in grams and mixed into clear rather than sprayed as a standalone powder.
+    /// Null/empty for standard powders. Derived at import from the vendor's taxonomy, NOT stored
+    /// from their raw category list.
+    /// </summary>
+    public string? Category { get; set; }
+
+    /// <summary>
+    /// Provenance of this record — e.g. "Columbia Coatings API". Kept SEPARATE from
+    /// <see cref="VendorName"/> (which holds the derived manufacturer) so we can honor a
+    /// distributor's right-to-delete by purging every record that came from their feed,
+    /// regardless of which manufacturer made the product.
+    /// </summary>
+    public string? Source { get; set; }
+
+    /// <summary>
+    /// Reformulation history as supplied by the vendor — e.g. "Formulation Change: 05/22/26".
+    /// Not a reliable modified-date (free text, reformulations only) but a useful signal that a
+    /// product's formula — and therefore its cure specs — may have changed.
+    /// </summary>
+    public string? FormulationChanges { get; set; }
+
     /// <summary>True when the vendor has discontinued this product. Kept for historical lookups.</summary>
     public bool IsDiscontinued { get; set; } = false;