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/CustomItemTemplate.cs

@@ -35,4 +35,19 @@ public class CustomItemTemplate : BaseEntity
     /// Path format: {companyId}/{templateId}/diagram.{ext}
     /// </summary>
     public string? DiagramImagePath { get; set; }
+
+    // ── Community library tracking ─────────────────────────────────────────
+
+    /// <summary>
+    /// Set when this template was imported from the community library.
+    /// Null for originally created templates.
+    /// </summary>
+    public int? SourceFormulaLibraryItemId { get; set; }
+    public virtual FormulaLibraryItem? SourceFormulaLibraryItem { get; set; }
+
+    /// <summary>
+    /// True once the user edits an imported template. Only modified imports (and original
+    /// creations) are eligible to be shared back to the community library.
+    /// </summary>
+    public bool IsModifiedFromSource { get; set; }
 }

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!;
+}

src/PowderCoating.Core/Entities/FormulaLibraryItem.cs

@@ -0,0 +1,70 @@
+namespace PowderCoating.Core.Entities;
+
+/// <summary>
+/// Platform-level community library entry for a shared custom formula template.
+/// Not tenant-scoped — no BaseEntity, no CompanyId, no soft delete.
+/// Shared voluntarily by the originating company; imported as independent copies by others.
+/// </summary>
+public class FormulaLibraryItem
+{
+    public int Id { get; set; }
+
+    // ── Formula content (copied from CustomItemTemplate at share time) ─────
+
+    public string Name { get; set; } = string.Empty;
+    public string? Description { get; set; }
+
+    /// <summary>"FixedRate" or "SurfaceAreaSqFt" — mirrors CustomItemTemplate.OutputMode.</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;
+
+    public decimal? DefaultRate { get; set; }
+    public string? RateLabel { get; set; }
+    public string? Notes { get; set; }
+
+    /// <summary>
+    /// Blob path referencing the source template's diagram image.
+    /// Nulled out (here and on all imports) if the source template's diagram is removed.
+    /// </summary>
+    public string? DiagramImagePath { get; set; }
+
+    // ── Attribution ────────────────────────────────────────────────────────
+
+    /// <summary>Comma-separated community tags, e.g. "HVAC,Sheet Metal".</summary>
+    public string? Tags { get; set; }
+
+    /// <summary>Optional industry hint shown on the browse card, e.g. "HVAC", "Automotive".</summary>
+    public string? IndustryHint { get; set; }
+
+    /// <summary>Id of the CustomItemTemplate this was shared from.</summary>
+    public int SourceCustomItemTemplateId { get; set; }
+
+    public int SourceCompanyId { get; set; }
+
+    /// <summary>Denormalized company name so it renders without a join when the company is gone.</summary>
+    public string SourceCompanyName { get; set; } = string.Empty;
+
+    /// <summary>
+    /// When non-null, this entry was derived from an imported formula that was subsequently
+    /// modified. Points to the original library entry. Shown as "Inspired by..." on the browse card.
+    /// </summary>
+    public int? InspiredByFormulaLibraryItemId { get; set; }
+    public virtual FormulaLibraryItem? InspiredBy { get; set; }
+
+    public string SharedByUserId { get; set; } = string.Empty;
+    public DateTime SharedAt { get; set; } = DateTime.UtcNow;
+
+    /// <summary>False when the creator has removed it from the community library.</summary>
+    public bool IsPublished { get; set; } = true;
+
+    /// <summary>Running count of how many companies have imported this entry.</summary>
+    public int ImportCount { get; set; }
+
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    public DateTime? UpdatedAt { get; set; }
+}