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/CustomFieldService.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace App\Services;
+
+use App\Models\CustomField;
+use Illuminate\Http\Request;
+
+class CustomFieldService
+{
+    public function create(Request $request): CustomField
+    {
+        $data = $request->validated();
+        $data[getCustomFieldValueKey($request->type)] = $request->default_answer;
+        $data['company_id'] = $request->header('company');
+        $data['slug'] = clean_slug($request->model_type, $request->name);
+
+        return CustomField::create($data);
+    }
+
+    public function update(CustomField $customField, Request $request): CustomField
+    {
+        $data = $request->validated();
+        $data[getCustomFieldValueKey($request->type)] = $request->default_answer;
+        $customField->update($data);
+
+        return $customField;
+    }
+}