Refactor exchange rate providers into driver-based architecture

Replace duplicated switch/case blocks across 4 methods with a clean
abstract driver pattern:

- ExchangeRateDriver (abstract): defines getExchangeRate(),
  getSupportedCurrencies(), validateConnection()
- CurrencyFreakDriver, CurrencyLayerDriver, OpenExchangeRateDriver,
  CurrencyConverterDriver: concrete implementations
- ExchangeRateDriverFactory: resolves driver name to class, with
  register() method for module extensibility

Delete ExchangeRateProvidersTrait — all logic now lives in driver
classes and ExchangeRateProviderService. Adding a new exchange rate
provider only requires implementing ExchangeRateDriver and calling
ExchangeRateDriverFactory::register() in a module service provider.

app/Services/ExchangeRate/ExchangeRateDriverFactory.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace App\Services\ExchangeRate;
+
+use InvalidArgumentException;
+
+class ExchangeRateDriverFactory
+{
+    /** @var array<string, class-string<ExchangeRateDriver>> */
+    protected static array $drivers = [
+        'currency_freak' => CurrencyFreakDriver::class,
+        'currency_layer' => CurrencyLayerDriver::class,
+        'open_exchange_rate' => OpenExchangeRateDriver::class,
+        'currency_converter' => CurrencyConverterDriver::class,
+    ];
+
+    /**
+     * Register a custom exchange rate driver (for module extensibility).
+     */
+    public static function register(string $name, string $driverClass): void
+    {
+        static::$drivers[$name] = $driverClass;
+    }
+
+    public static function make(string $driver, string $apiKey, array $config = []): ExchangeRateDriver
+    {
+        $class = static::$drivers[$driver] ?? null;
+
+        if (! $class) {
+            throw new InvalidArgumentException("Unknown exchange rate driver: {$driver}");
+        }
+
+        return new $class($apiKey, $config);
+    }
+
+    /**
+     * Get all registered driver names.
+     */
+    public static function availableDrivers(): array
+    {
+        return array_keys(static::$drivers);
+    }
+}