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/Pages/Upload.cshtml.cs

@@ -100,7 +100,7 @@ namespace MoneyMap.Pages
                 {
                     var categorizationResult = await _categorizer.CategorizeAsync(preview.Transaction.Name, preview.Transaction.Amount);
                     preview.Transaction.Category = categorizationResult.Category;
-                    preview.Transaction.Merchant = categorizationResult.Merchant;
+                    preview.Transaction.MerchantId = categorizationResult.MerchantId;
                     preview.SuggestedCategory = categorizationResult.Category;
                 }
             }