Refactor: Extract CategoryMapping model and add caching

- Move CategoryMapping class to Models/CategoryMapping.cs
- Add IMemoryCache with 10-minute TTL for category mappings
- Add InvalidateMappingsCache() method for cache invalidation
- Reduces repeated DB queries during bulk categorization

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

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

MoneyMap/Models/CategoryMapping.cs

@@ -0,0 +1,47 @@
+namespace MoneyMap.Models
+{
+    /// <summary>
+    /// Represents a mapping rule that associates transaction name patterns with categories.
+    /// Used for automatic categorization of transactions during import.
+    /// </summary>
+    public class CategoryMapping
+    {
+        public int Id { get; set; }
+
+        /// <summary>
+        /// The category to assign when a transaction matches the pattern.
+        /// </summary>
+        public required string Category { get; set; }
+
+        /// <summary>
+        /// The pattern to match against transaction names (case-insensitive contains).
+        /// </summary>
+        public required string Pattern { get; set; }
+
+        /// <summary>
+        /// Higher priority mappings are checked first. Default is 0.
+        /// </summary>
+        public int Priority { get; set; } = 0;
+
+        /// <summary>
+        /// Optional merchant to auto-assign when this pattern matches.
+        /// </summary>
+        public int? MerchantId { get; set; }
+        public Merchant? Merchant { get; set; }
+
+        /// <summary>
+        /// AI confidence score when this rule was created by AI (0.0 - 1.0).
+        /// </summary>
+        public decimal? Confidence { get; set; }
+
+        /// <summary>
+        /// Who created this rule: "User" or "AI".
+        /// </summary>
+        public string? CreatedBy { get; set; }
+
+        /// <summary>
+        /// When this rule was created.
+        /// </summary>
+        public DateTime? CreatedAt { get; set; }
+    }
+}

MoneyMap/Pages/CategoryMappings.cshtml.cs

@@ -2,6 +2,7 @@ using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.RazorPages;
 using Microsoft.EntityFrameworkCore;
 using MoneyMap.Data;
+using MoneyMap.Models;
 using MoneyMap.Services;
 using System.ComponentModel.DataAnnotations;
 using System.Text;

MoneyMap/Services/TransactionCategorizer.cs

@@ -1,34 +1,17 @@
 using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Caching.Memory;
 using MoneyMap.Data;
+using MoneyMap.Models;
 
 namespace MoneyMap.Services
 {
-    // ===== Models =====
-
-    public class CategoryMapping
-    {
-        public int Id { get; set; }
-        public required string Category { get; set; }
-        public required string Pattern { get; set; }
-        public int Priority { get; set; } = 0; // Higher priority = checked first
-
-        // Merchant relationship
-        public int? MerchantId { get; set; }
-        public Models.Merchant? Merchant { get; set; }
-
-        // AI categorization tracking
-        public decimal? Confidence { get; set; } // AI confidence score (0.0 - 1.0)
-        public string? CreatedBy { get; set; } // "User" or "AI"
-        public DateTime? CreatedAt { get; set; } // When rule was created
-    }
-
-    // ===== Service Interface =====
 
     public interface ITransactionCategorizer
     {
         Task<CategorizationResult> CategorizeAsync(string merchantName, decimal? amount = null);
         Task<List<CategoryMapping>> GetAllMappingsAsync();
         Task SeedDefaultMappingsAsync();
+        void InvalidateMappingsCache();
     }
 
     public class CategorizationResult
@@ -42,11 +25,20 @@ namespace MoneyMap.Services
     public class TransactionCategorizer : ITransactionCategorizer
     {
         private readonly MoneyMapContext _db;
+        private readonly IMemoryCache _cache;
         private const decimal GasStationThreshold = -20m;
+        private const string MappingsCacheKey = "CategoryMappings";
+        private static readonly TimeSpan CacheDuration = TimeSpan.FromMinutes(10);
 
-        public TransactionCategorizer(MoneyMapContext db)
+        public TransactionCategorizer(MoneyMapContext db, IMemoryCache cache)
         {
             _db = db;
+            _cache = cache;
+        }
+
+        public void InvalidateMappingsCache()
+        {
+            _cache.Remove(MappingsCacheKey);
         }
 
         public async Task<CategorizationResult> CategorizeAsync(string merchantName, decimal? amount = null)
@@ -56,11 +48,8 @@ namespace MoneyMap.Services
 
             var merchantUpper = merchantName.ToUpperInvariant();
 
-            // Get all mappings ordered by priority
-            var mappings = await _db.CategoryMappings
-                .OrderByDescending(m => m.Priority)
-                .ThenBy(m => m.Category)
-                .ToListAsync();
+            // Get cached mappings or load from database
+            var mappings = await GetCachedMappingsAsync();
 
             // Special case: Gas stations with small purchases
             if (amount.HasValue && amount.Value > GasStationThreshold)
@@ -91,12 +80,26 @@ namespace MoneyMap.Services
             return new CategorizationResult(); // No match - needs manual categorization
         }
 
+        private async Task<List<CategoryMapping>> GetCachedMappingsAsync()
+        {
+            if (_cache.TryGetValue(MappingsCacheKey, out List<CategoryMapping>? cachedMappings) && cachedMappings != null)
+            {
+                return cachedMappings;
+            }
+
+            var mappings = await _db.CategoryMappings
+                .OrderByDescending(m => m.Priority)
+                .ThenBy(m => m.Category)
+                .ToListAsync();
+
+            _cache.Set(MappingsCacheKey, mappings, CacheDuration);
+            return mappings;
+        }
+
         public async Task<List<CategoryMapping>> GetAllMappingsAsync()
         {
-            return await _db.CategoryMappings
-                .OrderBy(m => m.Category)
-                .ThenByDescending(m => m.Priority)
-                .ToListAsync();
+            var mappings = await GetCachedMappingsAsync();
+            return mappings.OrderBy(m => m.Category).ThenByDescending(m => m.Priority).ToList();
         }
 
         public async Task SeedDefaultMappingsAsync()