Make SimpleFIN split migration run only once

Track completion in a `data_migrations` table. On startup the
migration checks if it has already been applied and skips the
SimpleFIN API calls entirely, avoiding the slow startup.

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

src/Purrse.Api/Services/SimpleFinConnectionMigration.cs

@@ -10,13 +10,28 @@ namespace Purrse.Api.Services;
 /// <summary>
 /// One-time data migration that splits SimpleFIN connections containing accounts
 /// from multiple institutions into separate connections per institution.
+/// Tracks completion in a `data_migrations` table so it only runs once.
 /// </summary>
 public static class SimpleFinConnectionMigration
 {
+    private const string MigrationName = "SplitSimpleFinConnectionsByInstitution";
+
     public static async Task RunAsync(IServiceProvider services, ILogger logger)
     {
         using var scope = services.CreateScope();
         var db = scope.ServiceProvider.GetRequiredService<PurrseDbContext>();
+
+        // Create tracking table if it doesn't exist, then check if already applied
+        await db.Database.ExecuteSqlRawAsync(
+            "CREATE TABLE IF NOT EXISTS data_migrations (name TEXT PRIMARY KEY, applied_at TIMESTAMP NOT NULL)");
+
+        var alreadyApplied = await db.Database
+            .SqlQueryRaw<int>($"SELECT 1 AS \"Value\" FROM data_migrations WHERE name = '{MigrationName}'")
+            .AnyAsync();
+
+        if (alreadyApplied)
+            return;
+
         var encryption = scope.ServiceProvider.GetRequiredService<IEncryptionService>();
         var simpleFin = scope.ServiceProvider.GetRequiredService<SimpleFinSyncProvider>();
 
@@ -26,7 +41,10 @@ public static class SimpleFinConnectionMigration
             .ToListAsync();
 
         if (connections.Count == 0)
+        {
+            await MarkCompleteAsync(db);
             return;
+        }
 
         var splitCount = 0;
 
@@ -113,5 +131,13 @@ public static class SimpleFinConnectionMigration
             await db.SaveChangesAsync();
             logger.LogInformation("SimpleFIN connection migration complete: created {Count} new connections", splitCount);
         }
+
+        await MarkCompleteAsync(db);
+    }
+
+    private static async Task MarkCompleteAsync(PurrseDbContext db)
+    {
+        await db.Database.ExecuteSqlRawAsync(
+            $"INSERT INTO data_migrations (name, applied_at) VALUES ('{MigrationName}', NOW()) ON CONFLICT DO NOTHING");
     }
 }