fix(host): wait for configuration database before applying migrations

Central nodes crashed at startup with `CREATE DATABASE permission denied`
when MSSQL accepted connections before recovering user databases —
DB_ID(@db) returned null, so EF Core's MigrateAsync fell through to
SqlServerDatabaseCreator.CreateAsync. The non-privileged app login then
failed CREATE DATABASE and the host terminated with FTL, leaving Traefik's
/health/active probe unable to find an upstream ("no available server" at
localhost:9000).

Add MigrationHelper.WaitForDatabaseReadyAsync that polls
Database.CanConnectAsync() for up to 60s before invoking MigrateAsync, and
thread an ILogger through so retry attempts surface in normal logs. This
removes the startup race without requiring depends_on across compose stacks
or granting dbcreator to the app login.

src/ScadaLink.Host/Program.cs

@@ -109,7 +109,10 @@ try
             using (var scope = app.Services.CreateScope())
             {
                 var dbContext = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
-                await MigrationHelper.ApplyOrValidateMigrationsAsync(dbContext, isDevelopment);
+                var migrationLogger = scope.ServiceProvider
+                    .GetRequiredService<ILoggerFactory>()
+                    .CreateLogger(typeof(MigrationHelper).FullName!);
+                await MigrationHelper.ApplyOrValidateMigrationsAsync(dbContext, isDevelopment, migrationLogger);
             }
         }