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/CustomItemTemplate.cs

@@ -0,0 +1,38 @@
+namespace PowderCoating.Core.Entities;
+
+/// <summary>
+/// A per-company reusable pricing formula template. Users define named input fields and an
+/// NCalc expression that produces either a fixed dollar amount (FixedRate) or a surface area
+/// in square feet (SurfaceAreaSqFt) that feeds the standard coatings pricing path.
+/// </summary>
+public class CustomItemTemplate : BaseEntity
+{
+    public string Name { get; set; } = string.Empty;
+    public string? Description { get; set; }
+
+    /// <summary>"FixedRate" or "SurfaceAreaSqFt" — controls which pricing path is used after evaluation.</summary>
+    public string OutputMode { get; set; } = "FixedRate";
+
+    /// <summary>JSON array of field definitions: [{name, label, unit, defaultValue}]</summary>
+    public string FieldsJson { get; set; } = "[]";
+
+    /// <summary>NCalc expression using field name slugs and the reserved variable 'rate'.</summary>
+    public string Formula { get; set; } = string.Empty;
+
+    /// <summary>Default rate value populated into the quote wizard; user can override per quote.</summary>
+    public decimal? DefaultRate { get; set; }
+
+    /// <summary>Display label for the rate field, e.g. "$/sq in" or "$/lb".</summary>
+    public string? RateLabel { get; set; }
+
+    public string? Notes { get; set; }
+    public int DisplayOrder { get; set; }
+    public bool IsActive { get; set; } = true;
+
+    /// <summary>
+    /// Optional reference diagram (shop drawing, sketch) stored in blob storage.
+    /// Shown in the template editor and quote wizard so users know which measurements to take.
+    /// Path format: {companyId}/{templateId}/diagram.{ext}
+    /// </summary>
+    public string? DiagramImagePath { get; set; }
+}

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; }

src/PowderCoating.Core/Entities/QuoteItem.cs

@@ -56,6 +56,14 @@ public class QuoteItem : 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 Quote Quote { get; set; } = null!;
     public virtual CatalogItem? CatalogItem { get; set; }