Add Community Formula Library feature

Companies can now share their custom formula templates to a platform-wide
community library. Other tenants can browse, preview, and import formulas
as independent local copies. Includes attribution (source company name),
"Inspired by" lineage for re-contributed formulas, import counts, own-formula
badge, cascade diagram nullification, and AI assistant + help docs updates.

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

src/PowderCoating.Core/Entities/FormulaLibraryImport.cs

@@ -0,0 +1,19 @@
+namespace PowderCoating.Core.Entities;
+
+/// <summary>
+/// Records that a company imported a specific FormulaLibraryItem into their local template library.
+/// Tenant-scoped via BaseEntity.CompanyId. One row per (company, library item) — re-importing the
+/// same item overwrites the existing row rather than creating a duplicate.
+/// </summary>
+public class FormulaLibraryImport : BaseEntity
+{
+    public int FormulaLibraryItemId { get; set; }
+    public virtual FormulaLibraryItem FormulaLibraryItem { get; set; } = null!;
+
+    public string ImportedByUserId { get; set; } = string.Empty;
+    public DateTime ImportedAt { get; set; } = DateTime.UtcNow;
+
+    /// <summary>The CustomItemTemplate row created in this company's local library on import.</summary>
+    public int ResultingCustomItemTemplateId { get; set; }
+    public virtual CustomItemTemplate ResultingCustomItemTemplate { get; set; } = null!;
+}