fix(transactions): annotate Transaction::$source enum type for static analysis

The source attribute is cast to TransactionSource but was undocumented, so
Larastan inferred it as string and flagged the enum comparison in
UpdateTransactionRequest as always-false. Document the cast type like the
sibling category_source property.

app/Models/Transaction.php

@@ -26,6 +26,7 @@ use Illuminate\Database\Eloquent\SoftDeletes;
 /**
  * @property Carbon $transaction_date
  * @property int|float $total_amount
+ * @property TransactionSource $source
  * @property ?CategorySource $category_source
  * @property ?float $ai_confidence
  * @property ?string $categorized_by_rule_id