Feature: expand MerchantService with full CRUD operations

Extend MerchantService with additional operations:
- GetMerchantByIdAsync: Retrieve merchant with optional related data
- GetAllMerchantsWithStatsAsync: Get all merchants with transaction/mapping counts
- UpdateMerchantAsync: Update merchant name with duplicate validation
- DeleteMerchantAsync: Delete merchant (unlinks transactions and mappings)

Includes DTOs for merchant stats, update results, and delete results. Consolidates merchant management logic previously scattered across PageModels.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

MoneyMap/Services/MerchantService.cs

@@ -9,6 +9,10 @@ namespace MoneyMap.Services
         Task<Merchant?> FindByNameAsync(string name);
         Task<Merchant> GetOrCreateAsync(string name);
         Task<int?> GetOrCreateIdAsync(string? name);
+        Task<Merchant?> GetMerchantByIdAsync(int id, bool includeRelated = false);
+        Task<List<MerchantWithStats>> GetAllMerchantsWithStatsAsync();
+        Task<MerchantUpdateResult> UpdateMerchantAsync(int id, string newName);
+        Task<MerchantDeleteResult> DeleteMerchantAsync(int id);
     }
 
     public class MerchantService : IMerchantService
@@ -54,5 +58,127 @@ namespace MoneyMap.Services
             var merchant = await GetOrCreateAsync(name);
             return merchant.Id;
         }
+
+        public async Task<Merchant?> GetMerchantByIdAsync(int id, bool includeRelated = false)
+        {
+            var query = _db.Merchants.AsQueryable();
+
+            if (includeRelated)
+            {
+                query = query
+                    .Include(m => m.Transactions)
+                    .Include(m => m.CategoryMappings);
+            }
+
+            return await query.FirstOrDefaultAsync(m => m.Id == id);
+        }
+
+        public async Task<List<MerchantWithStats>> GetAllMerchantsWithStatsAsync()
+        {
+            var merchants = await _db.Merchants
+                .Include(m => m.Transactions)
+                .Include(m => m.CategoryMappings)
+                .OrderBy(m => m.Name)
+                .ToListAsync();
+
+            return merchants.Select(m => new MerchantWithStats
+            {
+                Id = m.Id,
+                Name = m.Name,
+                TransactionCount = m.Transactions.Count,
+                MappingCount = m.CategoryMappings.Count
+            }).ToList();
+        }
+
+        public async Task<MerchantUpdateResult> UpdateMerchantAsync(int id, string newName)
+        {
+            var merchant = await _db.Merchants.FindAsync(id);
+            if (merchant == null)
+            {
+                return new MerchantUpdateResult
+                {
+                    Success = false,
+                    Message = "Merchant not found."
+                };
+            }
+
+            var trimmedName = newName.Trim();
+
+            // Check if another merchant with the same name exists
+            var existing = await _db.Merchants
+                .FirstOrDefaultAsync(m => m.Name == trimmedName && m.Id != id);
+
+            if (existing != null)
+            {
+                return new MerchantUpdateResult
+                {
+                    Success = false,
+                    Message = $"Merchant '{trimmedName}' already exists."
+                };
+            }
+
+            merchant.Name = trimmedName;
+            await _db.SaveChangesAsync();
+
+            return new MerchantUpdateResult
+            {
+                Success = true,
+                Message = "Merchant updated successfully."
+            };
+        }
+
+        public async Task<MerchantDeleteResult> DeleteMerchantAsync(int id)
+        {
+            var merchant = await _db.Merchants
+                .Include(m => m.Transactions)
+                .Include(m => m.CategoryMappings)
+                .FirstOrDefaultAsync(m => m.Id == id);
+
+            if (merchant == null)
+            {
+                return new MerchantDeleteResult
+                {
+                    Success = false,
+                    Message = "Merchant not found."
+                };
+            }
+
+            var transactionCount = merchant.Transactions.Count;
+            var mappingCount = merchant.CategoryMappings.Count;
+
+            _db.Merchants.Remove(merchant);
+            await _db.SaveChangesAsync();
+
+            return new MerchantDeleteResult
+            {
+                Success = true,
+                Message = $"Deleted merchant '{merchant.Name}'. {transactionCount} transactions and {mappingCount} category mappings are now unlinked.",
+                TransactionCount = transactionCount,
+                MappingCount = mappingCount
+            };
+        }
+    }
+
+    // DTOs
+    public class MerchantWithStats
+    {
+        public int Id { get; set; }
+        public string Name { get; set; } = "";
+        public int TransactionCount { get; set; }
+        public int MappingCount { get; set; }
+    }
+
+    public class MerchantUpdateResult
+    {
+        public bool Success { get; set; }
+        public string Message { get; set; } = "";
+    }
+
+    public class MerchantDeleteResult
+    {
+        public bool Success { get; set; }
+        public string Message { get; set; } = "";
+        public int TransactionCount { get; set; }
+        public int MappingCount { get; set; }
     }
 }