feat(api): add MerchantsController with list and merge endpoints

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

MoneyMap/Controllers/MerchantsController.cs

@@ -0,0 +1,97 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.EntityFrameworkCore;
+using MoneyMap.Data;
+
+namespace MoneyMap.Controllers;
+
+[ApiController]
+[Route("api/[controller]")]
+public class MerchantsController : ControllerBase
+{
+    private readonly MoneyMapContext _db;
+
+    public MerchantsController(MoneyMapContext db) => _db = db;
+
+    [HttpGet]
+    public async Task<IActionResult> List([FromQuery] string? query = null)
+    {
+        var q = _db.Merchants
+            .Include(m => m.Transactions)
+            .Include(m => m.CategoryMappings)
+            .AsQueryable();
+
+        if (!string.IsNullOrWhiteSpace(query))
+            q = q.Where(m => m.Name.Contains(query));
+
+        var merchants = await q
+            .OrderBy(m => m.Name)
+            .Select(m => new
+            {
+                m.Id,
+                m.Name,
+                TransactionCount = m.Transactions.Count,
+                MappingCount = m.CategoryMappings.Count,
+                Categories = m.CategoryMappings.Select(cm => cm.Category).Distinct().ToList()
+            })
+            .ToListAsync();
+
+        return Ok(new { Count = merchants.Count, Merchants = merchants });
+    }
+
+    [HttpPost("merge")]
+    public async Task<IActionResult> Merge([FromBody] MergeMerchantsRequest request)
+    {
+        if (request.SourceMerchantId == request.TargetMerchantId)
+            return BadRequest(new { message = "Source and target merchant cannot be the same" });
+
+        var source = await _db.Merchants.FindAsync(request.SourceMerchantId);
+        var target = await _db.Merchants.FindAsync(request.TargetMerchantId);
+
+        if (source == null)
+            return NotFound(new { message = $"Source merchant {request.SourceMerchantId} not found" });
+        if (target == null)
+            return NotFound(new { message = $"Target merchant {request.TargetMerchantId} not found" });
+
+        var transactions = await _db.Transactions
+            .Where(t => t.MerchantId == request.SourceMerchantId)
+            .ToListAsync();
+
+        foreach (var t in transactions)
+            t.MerchantId = request.TargetMerchantId;
+
+        var sourceMappings = await _db.CategoryMappings
+            .Where(cm => cm.MerchantId == request.SourceMerchantId)
+            .ToListAsync();
+
+        var targetMappingPatterns = await _db.CategoryMappings
+            .Where(cm => cm.MerchantId == request.TargetMerchantId)
+            .Select(cm => cm.Pattern)
+            .ToListAsync();
+
+        foreach (var mapping in sourceMappings)
+        {
+            if (targetMappingPatterns.Contains(mapping.Pattern))
+                _db.CategoryMappings.Remove(mapping);
+            else
+                mapping.MerchantId = request.TargetMerchantId;
+        }
+
+        _db.Merchants.Remove(source);
+        await _db.SaveChangesAsync();
+
+        return Ok(new
+        {
+            Merged = true,
+            Source = new { source.Id, source.Name },
+            Target = new { target.Id, target.Name },
+            TransactionsReassigned = transactions.Count,
+            MappingsReassigned = sourceMappings.Count
+        });
+    }
+}
+
+public class MergeMerchantsRequest
+{
+    public int SourceMerchantId { get; set; }
+    public int TargetMerchantId { get; set; }
+}