feat(exchange-rate): make providers extendible via module Registry

Exchange rate providers are now pluggable via the module Registry. The four built-in drivers (currency_converter, currency_freak, currency_layer, open_exchange_rate) move from a static config array into App\\Providers\\DriverRegistryProvider, which calls Registry::registerExchangeRateDriver() for each during app boot with metadata the frontend needs: label (i18n key), website (help-text URL), and config_fields (schema for driver-specific driver_config JSON).

The Currency Converter's server-type selector and dedicated URL field — previously hardcoded in ExchangeRateProviderModal.vue — are now just another config_fields entry with a visible_when rule that shows the URL input only when type=DEDICATED. Any module that wants to ship a custom driver gets the same treatment for free: declare config_fields in the registration, and the host app's modal renders them automatically.

ExchangeRateDriverFactory::make() falls back to Registry::driverMeta() when a name isn't in the local built-in map, and availableDrivers() merges both sources. ConfigController handles the exchange_rate_drivers key specially by mapping Registry::allDrivers('exchange_rate') to enriched option objects, so the config-file route still works for every other key. The static exchange_rate_drivers + currency_converter_servers arrays in config/invoiceshelf.php are deleted.

Unit tests cover the new Registry::register/flushDrivers, the factory merging built-ins with Registry-contributed drivers, and the factory rejecting unknown names. A feature test exercises the end-to-end /api/v1/config?key=exchange_rate_drivers response shape.

NOTE: this commit depends on invoiceshelf/modules package commit e44d951 which adds the Registry driver API. The package needs to be released and pinned in composer.json before a fresh composer install on this commit will work.

app/Services/ExchangeRate/ExchangeRateDriverFactory.php

@@ -3,10 +3,20 @@
 namespace App\Services\ExchangeRate;
 
 use InvalidArgumentException;
+use InvoiceShelf\Modules\Registry;
 
 class ExchangeRateDriverFactory
 {
-    /** @var array<string, class-string<ExchangeRateDriver>> */
+    /**
+     * Built-in driver fallback map.
+     *
+     * Kept as a backstop so that direct calls to register() (without going through
+     * the module Registry) continue to work. Built-in drivers are also registered
+     * via the Registry by DriverRegistryProvider — that registration is the
+     * canonical source for driver metadata (label, website, config_fields).
+     *
+     * @var array<string, class-string<ExchangeRateDriver>>
+     */
     protected static array $drivers = [
         'currency_freak' => CurrencyFreakDriver::class,
         'currency_layer' => CurrencyLayerDriver::class,
@@ -15,7 +25,11 @@ class ExchangeRateDriverFactory
     ];
 
     /**
-     * Register a custom exchange rate driver (for module extensibility).
+     * Register a custom exchange rate driver directly with the factory.
+     *
+     * Modules should prefer Registry::registerExchangeRateDriver() instead, which
+     * carries metadata (label, website, config_fields) the frontend needs to render
+     * a configuration form for the driver.
      */
     public static function register(string $name, string $driverClass): void
     {
@@ -24,7 +38,7 @@ class ExchangeRateDriverFactory
 
     public static function make(string $driver, string $apiKey, array $config = []): ExchangeRateDriver
     {
-        $class = static::$drivers[$driver] ?? null;
+        $class = static::resolveDriverClass($driver);
 
         if (! $class) {
             throw new InvalidArgumentException("Unknown exchange rate driver: {$driver}");
@@ -34,10 +48,32 @@ class ExchangeRateDriverFactory
     }
 
     /**
-     * Get all registered driver names.
+     * Get all known driver names — both built-in/factory-registered and Registry-registered.
+     *
+     * @return array<int, string>
      */
     public static function availableDrivers(): array
     {
-        return array_keys(static::$drivers);
+        $local = array_keys(static::$drivers);
+        $registry = array_keys(Registry::allDrivers('exchange_rate'));
+
+        return array_values(array_unique(array_merge($local, $registry)));
+    }
+
+    /**
+     * Resolve a driver name to its concrete class.
+     *
+     * Checks the local $drivers map first (built-ins and factory::register() calls),
+     * then falls back to the module Registry.
+     */
+    protected static function resolveDriverClass(string $driver): ?string
+    {
+        if (isset(static::$drivers[$driver])) {
+            return static::$drivers[$driver];
+        }
+
+        $meta = Registry::driverMeta('exchange_rate', $driver);
+
+        return $meta['class'] ?? null;
     }
 }