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.Infrastructure/Data/ApplicationDbContext.cs

@@ -289,6 +289,12 @@ public class ApplicationDbContext : IdentityDbContext<ApplicationUser>, IDataPro
     /// </summary>
     public DbSet<PowderCatalogItem> PowderCatalogItems { get; set; }
 
+    /// <summary>Community library of shared formula templates. Platform-level, no tenant filter.</summary>
+    public DbSet<FormulaLibraryItem> FormulaLibraryItems { get; set; }
+
+    /// <summary>Per-company record of which community library formulas a company has imported.</summary>
+    public DbSet<FormulaLibraryImport> FormulaLibraryImports { get; set; }
+
     /// <summary>User-submitted bug reports; tenant-filtered with soft delete.</summary>
     public DbSet<BugReport> BugReports { get; set; }
     /// <summary>File attachments for bug reports; soft-delete only (no tenant filter — access controlled via parent BugReport).</summary>
@@ -2074,6 +2080,49 @@ modelBuilder.Entity<Job>()
             .HasIndex(c => new { c.CompanyId, c.MemoNumber })
             .IsUnique()
             .HasDatabaseName("IX_CreditMemos_CompanyId_MemoNumber");
+
+        // FormulaLibraryItem — platform-level, no tenant filter, no soft delete
+        // Self-referential "Inspired by" FK uses NoAction; cascade nullification handled in service.
+        modelBuilder.Entity<FormulaLibraryItem>()
+            .HasOne(f => f.InspiredBy)
+            .WithMany()
+            .HasForeignKey(f => f.InspiredByFormulaLibraryItemId)
+            .IsRequired(false)
+            .OnDelete(DeleteBehavior.NoAction);
+
+        modelBuilder.Entity<FormulaLibraryItem>()
+            .HasIndex(f => f.SourceCompanyId)
+            .HasDatabaseName("IX_FormulaLibraryItems_SourceCompanyId");
+
+        modelBuilder.Entity<FormulaLibraryItem>()
+            .HasIndex(f => f.IsPublished)
+            .HasDatabaseName("IX_FormulaLibraryItems_IsPublished");
+
+        // FormulaLibraryImport — tenant-scoped; unique per (CompanyId, FormulaLibraryItemId)
+        modelBuilder.Entity<FormulaLibraryImport>()
+            .HasOne(i => i.FormulaLibraryItem)
+            .WithMany()
+            .HasForeignKey(i => i.FormulaLibraryItemId)
+            .OnDelete(DeleteBehavior.Restrict);
+
+        modelBuilder.Entity<FormulaLibraryImport>()
+            .HasOne(i => i.ResultingCustomItemTemplate)
+            .WithMany()
+            .HasForeignKey(i => i.ResultingCustomItemTemplateId)
+            .OnDelete(DeleteBehavior.Restrict);
+
+        modelBuilder.Entity<FormulaLibraryImport>()
+            .HasIndex(i => new { i.CompanyId, i.FormulaLibraryItemId })
+            .IsUnique()
+            .HasDatabaseName("IX_FormulaLibraryImports_Company_Item");
+
+        // CustomItemTemplate → FormulaLibraryItem (nullable; only set on imported templates)
+        modelBuilder.Entity<CustomItemTemplate>()
+            .HasOne(t => t.SourceFormulaLibraryItem)
+            .WithMany()
+            .HasForeignKey(t => t.SourceFormulaLibraryItemId)
+            .IsRequired(false)
+            .OnDelete(DeleteBehavior.SetNull);
     }
 
     /// <summary>