Add subscription status helpers

status() returns draft, trial, active, paused, cancelling, expired or
cancelled (SubscriptionStatus constants), derived from the fields the
same way Anikeen ID derives its status, so it also works with older
responses. isActive() is true while the subscription runs and is paid.
Both Subscription and AppSubscription have them. Document the states.

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

src/Id/Enums/SubscriptionStatus.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace Anikeen\Id\Enums;
+
+/**
+ * The state of a subscription, see Subscription::status() and AppSubscription::status().
+ */
+class SubscriptionStatus
+{
+    /** Created, not checked out or confirmed by the user yet. */
+    const DRAFT = 'draft';
+
+    /** Running and billed, in its trial period. */
+    const TRIAL = 'trial';
+
+    /** Running and billed. */
+    const ACTIVE = 'active';
+
+    /** Paused: runs until its paid period ends, then expires. */
+    const PAUSED = 'paused';
+
+    /** Running until a scheduled cancellation takes effect. */
+    const CANCELLING = 'cancelling';
+
+    /** Paused and ran out: can be resumed (new period) or cancelled. */
+    const EXPIRED = 'expired';
+
+    /** Cancelled for good. */
+    const CANCELLED = 'cancelled';
+
+    /** None of the above. */
+    const INACTIVE = 'inactive';
+}