Add Custom Formula Item Templates with AI generation and wizard integration

Introduces per-company reusable NCalc2 pricing formula templates for complex
fabricated items (roof curbs, enclosures, welded frames). Templates support
two output modes — FixedRate (formula yields a dollar amount) and SurfaceAreaSqFt
(formula yields sq ft fed into the standard coating engine). Includes:

- CustomItemTemplate entity, migration (AddCustomItemTemplates), IUnitOfWork repo
- IsCustomFormulaItem / CustomItemTemplateId / FormulaFieldValuesJson flags on
  QuoteItem, JobItem, CreateQuoteItemDto; mapped in all 3 JobItemAssemblyService
  overloads and all existingItemsData JSON projections + pageMeta blocks
- ICustomFormulaAiService / CustomFormulaAiService: Claude-powered formula
  generator (natural language + optional diagram image) and NCalc2 evaluator
- CompanySettings CRUD endpoints: GetCustomItemTemplates, Create/Update/Delete,
  UploadTemplateDiagram, TemplateDiagram (blob serve), EvaluateFormula, GenerateFormulaFromAi
- Company Settings "Custom Formulas" tab + cfModal + company-settings-custom-formulas.js
- item-wizard.js: formula item type card, renderFormulaFields, wzFormulaRecalc
  (live evaluate via POST), collectStep2 formula branch, buildCardHtml / emitHiddenFields
- Formula badge in Quotes/Details and Jobs/Details; AI badge gap fixed in Jobs/Details
- Help article (CustomFormulaTemplates.cshtml), Help Index card, HelpController action,
  HelpKnowledgeBase entry; 225/225 unit tests passing

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

src/PowderCoating.Core/Entities/JobItem.cs

@@ -52,6 +52,14 @@ public class JobItem : BaseEntity
     public int? AiPredictionId { get; set; }
     public virtual AiItemPrediction? AiPrediction { get; set; }
 
+    // Custom formula item — see IsCustomFormulaItem routing in PricingCalculationService
+    public bool IsCustomFormulaItem { get; set; }
+    public int? CustomItemTemplateId { get; set; }
+    public virtual CustomItemTemplate? CustomItemTemplate { get; set; }
+
+    /// <summary>Snapshot of field name/value pairs used in the formula, stored as JSON for display on details views.</summary>
+    public string? FormulaFieldValuesJson { get; set; }
+
     // Relationships
     public virtual Job Job { get; set; } = null!;
     public virtual CatalogItem? CatalogItem { get; set; }