Transaction Model: explicitly cast decimal to string

Laravel defers to PDO & the underlying database as to what type decimals are cast as.
When using sqlite text that match a float will be returned as a float, while mySql always returns a string for a decimal.
This leads to crashes with sqlite while trying to import tranansactions. (It does not happen every transaction, only when the balance{before,after} coming from the database is floatable)

`FireflyIII\Support\Models\AccountBalanceCalculator::getLatesBalance(): Return value must be of type string, float returned.`

Fixes: #9458

Signed-off-by: yparitcher <y@paritcher.com>

app/Models/Transaction.php

@@ -54,6 +54,8 @@ class Transaction extends Model
             'bill_name_encrypted' => 'boolean',
             'reconciled'          => 'boolean',
             'balance_dirty'       => 'boolean',
+            'balance_before'      => 'string'
+            'balance_after'       => 'string'
             'date'                => 'datetime',
         ];