Convert merchant from string to entity with foreign keys

This refactors the merchant field from a simple string column to a normalized
entity with proper foreign key relationships:

**Database Changes:**
- Created Merchant entity/table with unique Name constraint
- Replaced Transaction.Merchant (string) with Transaction.MerchantId (FK)
- Replaced CategoryMapping.Merchant (string) with CategoryMapping.MerchantId (FK)
- Added proper foreign key constraints with SET NULL on delete
- Added indexes on MerchantId columns for performance

**Backend Changes:**
- Created MerchantService for finding/creating merchants
- Updated CategorizationResult to return MerchantId instead of merchant name
- Modified TransactionCategorizer to return MerchantId from pattern matches
- Updated Upload, Recategorize, and CategoryMappings to use merchant service
- Updated OpenAIReceiptParser to create/link merchants from parsed receipts
- Registered IMerchantService in dependency injection

**UI Changes:**
- Updated CategoryMappings UI to handle merchant entities (display as Merchant.Name)
- Updated Transactions page merchant filter to query by merchant entity
- Modified category mapping add/edit/import to create merchants on-the-fly
- Updated JavaScript to pass merchant names for edit modal

**Migration:**
- ConvertMerchantToEntity migration handles schema conversion
- Drops old string columns and creates new FK relationships
- All existing merchant data is lost (acceptable for this refactoring)

**Benefits:**
- Database normalization - merchants stored once, referenced many times
- Referential integrity with foreign keys
- Easier merchant management (rename once, updates everywhere)
- Foundation for future merchant features (logos, categories, etc.)
- Improved query performance with proper indexes

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

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

MoneyMap/Data/MoneyMapContext.cs

@@ -19,6 +19,7 @@ namespace MoneyMap.Data
         public DbSet<ReceiptLineItem> ReceiptLineItems => Set<ReceiptLineItem>();
 
         public DbSet<CategoryMapping> CategoryMappings => Set<CategoryMapping>();
+        public DbSet<Merchant> Merchants => Set<Merchant>();
 
         protected override void OnModelCreating(ModelBuilder modelBuilder)
         {
@@ -74,6 +75,13 @@ namespace MoneyMap.Data
                  .HasForeignKey(x => x.AccountId)
                  .OnDelete(DeleteBehavior.Restrict)
                  .IsRequired(false);
+
+                // Merchant (optional). If a merchant is deleted, set to null.
+                e.HasOne(x => x.Merchant)
+                 .WithMany(m => m.Transactions)
+                 .HasForeignKey(x => x.MerchantId)
+                 .OnDelete(DeleteBehavior.SetNull)
+                 .IsRequired(false);
             });
 
             // ---------- TRANSFER ----------
@@ -159,11 +167,33 @@ namespace MoneyMap.Data
                  .OnDelete(DeleteBehavior.Cascade);
             });
 
+            // ---------- MERCHANT ----------
+            modelBuilder.Entity<Merchant>(e =>
+            {
+                e.Property(x => x.Name).HasMaxLength(100).IsRequired();
+                e.HasIndex(x => x.Name).IsUnique();
+            });
+
+            // ---------- CATEGORY MAPPING ----------
+            modelBuilder.Entity<CategoryMapping>(e =>
+            {
+                e.Property(x => x.Category).HasMaxLength(100).IsRequired();
+                e.Property(x => x.Pattern).HasMaxLength(200).IsRequired();
+
+                // Merchant (optional). If a merchant is deleted, set to null.
+                e.HasOne(x => x.Merchant)
+                 .WithMany(m => m.CategoryMappings)
+                 .HasForeignKey(x => x.MerchantId)
+                 .OnDelete(DeleteBehavior.SetNull)
+                 .IsRequired(false);
+            });
+
             // ---------- Extra SQL Server–friendly indexes ----------
             // Fast filtering by date/amount/category
             modelBuilder.Entity<Transaction>().HasIndex(x => x.Date);
             modelBuilder.Entity<Transaction>().HasIndex(x => x.Amount);
             modelBuilder.Entity<Transaction>().HasIndex(x => x.Category);
+            modelBuilder.Entity<Transaction>().HasIndex(x => x.MerchantId);
         }
     }
 }