App subscriptions: manage a user's subscriptions with the app's own token

- $user->appSubscriptions(): create (idempotency_key), find, all, checkout, revoke, pause,
  resume, update against /v1/client/subscriptions with a client credentials token, so apps don't
  depend on the user's login
- user id from the billable's anikeen_id (AnikeenId::useUserIdField())
- ApiException with status, stable error code, isNotFound/isConflict/isTemporary; network
  errors no longer escape as raw Guzzle exceptions
- rejected app tokens are renewed once; asApp() never changes the shared client's token
- tests (testbench + Guzzle mock) and CI

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Id/AnikeenId.php

@@ -58,6 +58,8 @@ class AnikeenId
      */
     private static string $refreshTokenField = 'anikeen_id_refresh_token';
 
+    private static string $userIdField = 'anikeen_id';
+
     /**
      * Guzzle is used to make http requests.
      */
@@ -143,6 +145,41 @@ class AnikeenId
         return self::$accessTokenField;
     }
 
+    /**
+     * Attribute of the billable model that holds the user's Anikeen ID (used by app requests,
+     * e.g. appSubscriptions(), which act on behalf of a user without the user's token).
+     */
+    public static function useUserIdField(string $userIdField): void
+    {
+        self::$userIdField = $userIdField;
+    }
+
+    public static function getUserIdField(): string
+    {
+        return self::$userIdField;
+    }
+
+    /**
+     * A copy that uses the app's own access token (client credentials) instead of a user's token.
+     * The shared instance keeps its token, so app rights never leak into other requests.
+     *
+     * @throws Exceptions\RequestFreshAccessTokenException
+     */
+    public function asApp(): static
+    {
+        return (clone $this)->withToken(app(Contracts\AppTokenRepository::class)->getAccessToken());
+    }
+
+    /**
+     * Replace the HTTP client, e.g. with one using a Guzzle MockHandler in tests.
+     */
+    public function setHttpClient(Client $client): static
+    {
+        $this->client = $client;
+
+        return $this;
+    }
+
     public static function getMode(): string
     {
         return config('services.anikeen.mode') ?: 'production';