feat(imports): import history and delete by import

Track imports in DB, show history in Data section, allow deleting
transactions of a specific import instead of clearing all.

backend/src/services/transactions.ts

@@ -171,5 +171,6 @@ export async function updateTransaction(
 
 export async function clearAllTransactions(): Promise<{ deleted: number }> {
   const result = await pool.query('DELETE FROM transactions RETURNING id');
+  await pool.query('DELETE FROM imports');
   return { deleted: result.rowCount ?? 0 };
 }