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>

phpunit.xml

@@ -1,29 +1,16 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<phpunit bootstrap="vendor/autoload.php"
-         backupGlobals="false"
-         backupStaticAttributes="false"
-         colors="true"
-         verbose="true"
-         convertErrorsToExceptions="true"
-         convertNoticesToExceptions="true"
-         convertWarningsToExceptions="true"
-         processIsolation="false"
-         stopOnFailure="false"
-         printerClass="\Codedungeon\PHPUnitPrettyResultPrinter\Printer">
-    <testsuites>
-        <testsuite name="Unit">
-            <directory>tests</directory>
-        </testsuite>
-    </testsuites>
-    <filter>
-        <whitelist processUncoveredFilesFromWhitelist="true">
-            <directory suffix=".php">src</directory>
-        </whitelist>
-    </filter>
-    <php>
-        <env name="ANIKEEN_ID_KEY" value="38"/>
-        <env name="ANIKEEN_ID_SECRET" value="2goNRF8x37HPVZVaa28ySZGVXJuksvxnxM7TcGzM"/>
-        <env name="ANIKEEN_ID_BASE_URL" value="https://id.anikeen.com"/>
-        <env name="CLIENT_ACCESS_TOKEN" value="eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJhdWQiOiIxNSIsImp0aSI6IjAxN2QxZDg0Y2MxNjAyZTYxOGZkNjYwZGViZWVjMDY4MTk2YmYzMDk1OGMzY2RiYzBjZmJkZWFjZjFhOTUxODQzZDU1YTk3OGY2YWIxY2YzIiwiaWF0IjoxNjY0NjIzMzA4LjQyODI4MSwibmJmIjoxNjY0NjIzMzA4LjQyODI4NCwiZXhwIjoxNjgwMzQ4MTA4LjQxNjc4MSwic3ViIjoiMzkiLCJzY29wZXMiOlsiYXBpIiwicmVhZF91c2VyIl0sImNsaWVudCI6eyJ0cnVzdGVkIjpmYWxzZX19.vxnzCaU4PpOrNVHa5AnGSS6gX_RCvxIERAnHFhjTrUzRafV9mr2Cvwd-BDVYoUr10wHvxa_TJSYfnAdDuhE-MEyDv13O3dL2XGTtJNa_Rg6L6CQ0JvC3wL-lWPvGPFax9pu-_lqbA3jm5B08hc3-7tq3f2nXcxjhtkqT6TTJv1-RCAppb2HCXiUDAqANzbhyInDjOH2WvFj1OGC_AI03J3W2KRWyeFLtUne8XKPFyr9XGcPuTrqogcuuXLeUt2kcf2bXbuIV1OlgIECrDiP1Ee0F2AzPs27ZVJ2z0R0JbT6AubKhGl5_Qi27cwjOH7hT2dmjcF1mLjzpN1uChLIdSnGSoStH8VzYHnHE2I8G-owW_aadG2UmGdnRY143q6g_28f3WIZNSucBSXkwFeS_t4fylsvpxhpjYJusf5qiEU_X3YbeawYMUCFUkSD2XTIypAqMJLNZQAeJ52eyL-9fln-Bv7n9v7K9G6ieR6Tm0tsJ1PRnaQi7rA1NTFwHoQmIOW9tfMycLzT7bgSoz3ra6Ez2J7ZNuWBZNKS0O-0YfSrAWyWK5U8YRfQuSVzP2VrIU63K6RGU2c284PfQGy11kgKUNQPykirb8p7MDQ8PwrxKaylBnD6hhDgjqEh2bfsr_43DfJA0R58L1HK3BmQnxgap0C77wK1e0yNlABpN28Q"/>
-    </php>
-</phpunit>
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true">
+    <testsuites>
+        <testsuite name="Unit">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+    <source>
+        <include>
+            <directory suffix=".php">src</directory>
+        </include>
+    </source>
+</phpunit>