Extract business logic from remaining models to services

New services:
- ExchangeRateProviderService: CRUD, API status checks, currency converter
  URL resolution (extracted 122 lines from ExchangeRateProvider model)
- FileDiskService: create, update, setAsDefault, validateCredentials
  (extracted 97 lines from FileDisk model)
- ItemService: create/update with tax handling (extracted from Item model)
- TransactionService: create/complete/fail (extracted from Transaction model)
- CustomFieldService: create/update with slug generation (extracted from
  CustomField model)

Controllers updated to use constructor-injected services:
ExchangeRateProviderController, DiskController, ItemsController,
CustomFieldsController.

app/Services/TransactionService.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace App\Services;
+
+use App\Facades\Hashids;
+use App\Models\Transaction;
+
+class TransactionService
+{
+    public function create(array $data): Transaction
+    {
+        $transaction = Transaction::create($data);
+        $transaction->unique_hash = Hashids::connection(Transaction::class)->encode($transaction->id);
+        $transaction->save();
+
+        return $transaction;
+    }
+
+    public function complete(Transaction $transaction): void
+    {
+        $transaction->status = Transaction::SUCCESS;
+        $transaction->save();
+    }
+
+    public function fail(Transaction $transaction): void
+    {
+        $transaction->status = Transaction::FAILED;
+        $transaction->save();
+    }
+}