feat(expense-categories): AI 建議 for group/sub name + 990 line

Add an AI assist button to the Edit/New Group (大項) and Subcategory
(小項) dialogs: the user enters a Chinese name, and the model refines
the Chinese, translates it to English, and suggests the matching IRS
Form 990 Part IX line. Suggestions surface in a confirm card; Apply
fills the Chinese name, English name, and 990 line fields.

Backend mirrors the existing expense-classification AI family but over
the Form 990 line catalog: IExpenseCategoryAiService + base (catalog
load, prompt, id validation) + Claude/Gemini providers + factory that
picks the provider from ChurchProfile.AiProvider. New write-gated
POST api/expense-categories/ai-suggest endpoint; sub-category requests
pass the parent group + its 990 line to bias the choice.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API/ROLAC.API/Controllers/ExpenseCategoriesController.cs

@@ -3,6 +3,7 @@ using Microsoft.AspNetCore.Mvc;
 using ROLAC.API.Authorization;
 using ROLAC.API.DTOs.Expense;
 using ROLAC.API.Services;
+using ROLAC.API.Services.Ai;
 
 namespace ROLAC.API.Controllers;
 
@@ -13,12 +14,30 @@ namespace ROLAC.API.Controllers;
 public class ExpenseCategoriesController : ControllerBase
 {
     private readonly IExpenseCategoryService _svc;
-    public ExpenseCategoriesController(IExpenseCategoryService svc) => _svc = svc;
+    private readonly IExpenseCategoryAiServiceFactory _aiFactory;
+    public ExpenseCategoriesController(IExpenseCategoryService svc, IExpenseCategoryAiServiceFactory aiFactory)
+    {
+        _svc = svc;
+        _aiFactory = aiFactory;
+    }
 
     [HttpGet]
     public async Task<IActionResult> GetAll([FromQuery] bool includeInactive = false)
         => Ok(await _svc.GetAllAsync(includeInactive));
 
+    // Suggest an English name + Form 990 line for a category being defined. Write-gated: category
+    // editing is finance/admin-only, unlike the member-facing expense-ai/assist endpoint.
+    [HttpPost("ai-suggest")]
+    [HasPermission(Modules.ExpenseCategories, PermissionActions.Write)]
+    public async Task<IActionResult> AiSuggest([FromBody] ExpenseCategoryAiRequest r, CancellationToken ct)
+    {
+        if (string.IsNullOrWhiteSpace(r.Name_zh) && string.IsNullOrWhiteSpace(r.Name_en))
+            return BadRequest("A name is required.");
+
+        var svc = await _aiFactory.ResolveAsync(ct);
+        return Ok(await svc.SuggestAsync(r, ct));
+    }
+
     [HttpPost("groups")]
     [HasPermission(Modules.ExpenseCategories, PermissionActions.Write)]
     public async Task<IActionResult> CreateGroup([FromBody] CreateExpenseGroupRequest r)