fix(site-runtime): add SQLite schema migration for backup_configuration column

Existing site databases created before the primary/backup data connections
feature lack the backup_configuration and failover_retry_count columns.
Added TryAddColumnAsync migration that runs on startup after table creation.

src/ScadaLink.SiteRuntime/Persistence/SiteStorageService.cs

@@ -101,9 +101,35 @@ public class SiteStorageService
         ";
         await command.ExecuteNonQueryAsync();
 
+        // Schema migrations — add columns that may not exist on older databases
+        await MigrateSchemaAsync(connection);
+
         _logger.LogInformation("Site SQLite storage initialized at {ConnectionString}", _connectionString);
     }
 
+    private async Task MigrateSchemaAsync(SqliteConnection connection)
+    {
+        // Add backup_configuration and failover_retry_count to data_connection_definitions
+        // (added in primary/backup data connections feature)
+        await TryAddColumnAsync(connection, "data_connection_definitions", "backup_configuration", "TEXT");
+        await TryAddColumnAsync(connection, "data_connection_definitions", "failover_retry_count", "INTEGER NOT NULL DEFAULT 3");
+    }
+
+    private async Task TryAddColumnAsync(SqliteConnection connection, string table, string column, string type)
+    {
+        try
+        {
+            await using var cmd = connection.CreateCommand();
+            cmd.CommandText = $"ALTER TABLE {table} ADD COLUMN {column} {type}";
+            await cmd.ExecuteNonQueryAsync();
+            _logger.LogInformation("Migrated: added column {Column} to {Table}", column, table);
+        }
+        catch (SqliteException ex) when (ex.Message.Contains("duplicate column"))
+        {
+            // Column already exists — no action needed
+        }
+    }
+
     // ── Deployed Configuration CRUD ──
 
     /// <summary>