Add account registration with signup policy acceptance

Implements #16: invite-only student self-registration through a front-end
page, accepting signup-scoped policies at account creation.

Policy domain:
- us_policies.acceptance_scope (signup/booking/both); Policy::appliesTo();
  PolicyRepository::findForScope(); scope threaded through PolicyService,
  the REST create, the admin controller, and the Policies form.
- PolicyAcceptance::REG_ACCOUNT (registration_id = the new user's ID).

Auth:
- Invite value object + InviteRepository; us_invites table.
- RegistrationController + Invites admin page (manage_students): invite an
  email, share the registration link, revoke.
- RegistrationPage ([us_student_register] shortcode): validates the invite
  token, collects name/password, renders signup-scoped published policies
  with required acceptance, creates the us_student user, records account-type
  acceptances, marks the invite accepted, and logs the user in.
- RoleManager: manage_students cap added to STUDIO_ADMIN_CAPS.

Invite-only is implemented; the us_registration_mode self_approval path is a
documented future seam.

Docs: docs/features/account-registration.md; policies.md updated.
Tests: tests/Unit/Auth/ (Invite, InviteRepository) plus Policy scope
updates. composer test (104), cs, and PHPStan level 6 all pass.

Refs #16

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/Unit/Policy/PolicyValueObjectsTest.php

@@ -17,12 +17,14 @@ class PolicyValueObjectsTest extends TestCase
             'title'              => 'Cancellation',
             'slug'               => 'cancellation',
             'current_version_id' => '9',
+            'acceptance_scope'   => Policy::SCOPE_BOTH,
         ]);
 
         self::assertSame(4, $policy->id);
         self::assertSame('cancellation', $policy->slug);
         self::assertSame(9, $policy->currentVersionId);
-        self::assertArrayHasKey('current_version_id', $policy->toArray());
+        self::assertSame(Policy::SCOPE_BOTH, $policy->acceptanceScope);
+        self::assertArrayHasKey('acceptance_scope', $policy->toArray());
     }
 
     public function testPolicyHandlesNullCurrentVersion(): void
@@ -32,11 +34,23 @@ class PolicyValueObjectsTest extends TestCase
             'title'              => 'Cancellation',
             'slug'               => 'cancellation',
             'current_version_id' => null,
+            'acceptance_scope'   => Policy::SCOPE_BOOKING,
         ]);
 
         self::assertNull($policy->currentVersionId);
     }
 
+    public function testPolicyAppliesToScopeAndBoth(): void
+    {
+        $signup = new Policy('Terms', 'terms', acceptanceScope: Policy::SCOPE_SIGNUP);
+        self::assertTrue($signup->appliesTo(Policy::SCOPE_SIGNUP));
+        self::assertFalse($signup->appliesTo(Policy::SCOPE_BOOKING));
+
+        $both = new Policy('Both', 'both', acceptanceScope: Policy::SCOPE_BOTH);
+        self::assertTrue($both->appliesTo(Policy::SCOPE_SIGNUP));
+        self::assertTrue($both->appliesTo(Policy::SCOPE_BOOKING));
+    }
+
     public function testPolicyVersionFromRowAndStatusHelper(): void
     {
         $version = PolicyVersion::fromRow((object) [