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/ItemService.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace App\Services;
+
+use App\Models\CompanySetting;
+use App\Models\Item;
+use Illuminate\Http\Request;
+use Illuminate\Support\Facades\Auth;
+
+class ItemService
+{
+    public function create(Request $request): Item
+    {
+        $data = $request->validated();
+        $data['company_id'] = $request->header('company');
+        $data['creator_id'] = Auth::id();
+        $data['currency_id'] = CompanySetting::getSetting('currency', $request->header('company'));
+        $item = Item::create($data);
+
+        if ($request->has('taxes')) {
+            foreach ($request->taxes as $tax) {
+                $item->tax_per_item = true;
+                $item->save();
+                $tax['company_id'] = $request->header('company');
+                $item->taxes()->create($tax);
+            }
+        }
+
+        return Item::with('taxes')->find($item->id);
+    }
+
+    public function update(Item $item, Request $request): Item
+    {
+        $item->update($request->validated());
+
+        $item->taxes()->delete();
+
+        if ($request->has('taxes')) {
+            foreach ($request->taxes as $tax) {
+                $item->tax_per_item = true;
+                $item->save();
+                $tax['company_id'] = $request->header('company');
+                $item->taxes()->create($tax);
+            }
+        }
+
+        return Item::with('taxes')->find($item->id);
+    }
+}