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/FileDiskService.php

@@ -0,0 +1,85 @@
+<?php
+
+namespace App\Services;
+
+use App\Models\FileDisk;
+use Illuminate\Http\Request;
+
+class FileDiskService
+{
+    public function create(Request $request): FileDisk
+    {
+        if ($request->set_as_default) {
+            $this->clearDefaults();
+        }
+
+        return FileDisk::create([
+            'credentials' => $request->credentials,
+            'name' => $request->name,
+            'driver' => $request->driver,
+            'set_as_default' => $request->set_as_default,
+            'company_id' => $request->header('company'),
+        ]);
+    }
+
+    public function update(FileDisk $disk, Request $request): FileDisk
+    {
+        $data = [
+            'credentials' => $request->credentials,
+            'name' => $request->name,
+            'driver' => $request->driver,
+        ];
+
+        if (! $disk->set_as_default) {
+            if ($request->set_as_default) {
+                $this->clearDefaults();
+            }
+
+            $data['set_as_default'] = $request->set_as_default;
+        }
+
+        $disk->update($data);
+
+        return $disk;
+    }
+
+    public function setAsDefault(FileDisk $disk): FileDisk
+    {
+        $this->clearDefaults();
+
+        $disk->set_as_default = true;
+        $disk->save();
+
+        return $disk;
+    }
+
+    public function validateCredentials(array $credentials, string $driver): bool
+    {
+        FileDisk::setFilesystem(collect($credentials), $driver);
+
+        $prefix = env('DYNAMIC_DISK_PREFIX', 'temp_');
+
+        try {
+            $root = '';
+            if ($driver == 'dropbox') {
+                $root = $credentials['root'].'/';
+            }
+            \Storage::disk($prefix.$driver)->put($root.'invoiceshelf_temp.text', 'Check Credentials');
+
+            if (\Storage::disk($prefix.$driver)->exists($root.'invoiceshelf_temp.text')) {
+                \Storage::disk($prefix.$driver)->delete($root.'invoiceshelf_temp.text');
+
+                return true;
+            }
+        } catch (\Exception $e) {
+            return false;
+        }
+
+        return false;
+    }
+
+    private function clearDefaults(): void
+    {
+        FileDisk::query()->update(['set_as_default' => false]);
+    }
+}