refactor: drop ActorType::VOLUNTEER pending volunteer role introduction

VOLUNTEER was reserved-but-unused. Resolver mapped non-admin
authenticated users to ORG_MEMBER because Crewli has no dedicated
volunteer Spatie role; volunteer-ness is behaviour (shift assignments),
not identity.

Dead enum cases are YAGNI violations under zero-compromise: a future
developer could use the case without realising no resolution path
leads to it, producing a silent no-op. Re-introduce alongside a real
volunteer role split when that lands (BACKLOG OBS-1).

ActorType keeps ORGANIZER_ADMIN, SUPER_ADMIN, PORTAL_TOKEN, ORG_MEMBER,
UNAUTHENTICATED. Tests at 1537, Larastan clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/tests/Feature/Observability/BindSentryContextTest.php

@@ -140,9 +140,8 @@ final class BindSentryContextTest extends TestCase
 
     public function test_org_member_authenticated_user_tags_actor_type_org_member(): void
     {
-        // Crewli has no `volunteer` Spatie role today; volunteers fall into
-        // org_member. The VOLUNTEER ActorType case is reserved for a future
-        // split — see ActorType::resolve() docblock.
+        // Volunteer-ness is behaviour in Crewli (shift assignments), not
+        // identity — non-admin authenticated users resolve to ORG_MEMBER.
         $user = User::factory()->create();
         $user->assignRole('org_member');