Add startup migration to split existing SimpleFIN connections by institution

Runs once at startup after DB migrations. For each SimpleFIN
connection, re-discovers accounts from the API, groups by
institution, and splits connections that contain accounts from
multiple banks. Existing sync state and account mappings are
preserved.

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

src/Purrse.Api/Program.cs

@@ -119,6 +119,9 @@ using (var scope = app.Services.CreateScope())
     await db.Database.MigrateAsync();
 }
 
+// One-time data migration: split SimpleFIN connections by institution
+await SimpleFinConnectionMigration.RunAsync(app.Services, app.Services.GetRequiredService<ILogger<Program>>());
+
 // Initialize plugin system
 var pluginService = app.Services.GetRequiredService<PluginService>();
 await pluginService.DiscoverAndLoadPluginsAsync();