Add JsonStringEnumConverter for proper enum serialization

Without this, enums serialize as integers (0, 1, 2) instead of
strings ("Debit", "Credit", "Transfer"). This caused synced
transactions to have numeric type values and broke editing.

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

src/Purrse.Api/Program.cs

@@ -100,7 +100,11 @@ builder.Services.AddHostedService<BankSyncBackgroundService>();
 // SignalR
 builder.Services.AddSignalR();
 
-builder.Services.AddControllers();
+builder.Services.AddControllers()
+    .AddJsonOptions(options =>
+    {
+        options.JsonSerializerOptions.Converters.Add(new System.Text.Json.Serialization.JsonStringEnumConverter());
+    });
 builder.Services.AddEndpointsApiExplorer();
 
 var app = builder.Build();