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 <[email protected]>

src/Policy/Policy.php

@@ -5,10 +5,22 @@ namespace Unsupervised\Schedular\Policy;
 
 class Policy {
 
+	public const SCOPE_SIGNUP  = 'signup';
+	public const SCOPE_BOOKING = 'booking';
+	public const SCOPE_BOTH    = 'both';
+
+	/**
+	 * All valid acceptance scopes — when a policy must be accepted.
+	 *
+	 * @var list<string>
+	 */
+	public const VALID_SCOPES = [ self::SCOPE_SIGNUP, self::SCOPE_BOOKING, self::SCOPE_BOTH ];
+
 	public function __construct(
 		public readonly string $title,
 		public readonly string $slug,
 		public readonly ?int $currentVersionId = null,
+		public readonly string $acceptanceScope = self::SCOPE_BOOKING,
 		public readonly ?int $id = null,
 	) {}
 
@@ -17,10 +29,18 @@ class Policy {
 			title:            $row->title,
 			slug:             $row->slug,
 			currentVersionId: null !== $row->current_version_id ? (int) $row->current_version_id : null,
+			acceptanceScope:  $row->acceptance_scope,
 			id:               (int) $row->id,
 		);
 	}
 
+	/**
+	 * Whether this policy must be accepted in the given gate (`signup` or `booking`).
+	 */
+	public function appliesTo( string $scope ): bool {
+		return $this->acceptanceScope === $scope || self::SCOPE_BOTH === $this->acceptanceScope;
+	}
+
 	/**
 	 * Returns a plain array representation of the policy.
 	 *
@@ -32,6 +52,7 @@ class Policy {
 			'title'              => $this->title,
 			'slug'               => $this->slug,
 			'current_version_id' => $this->currentVersionId,
+			'acceptance_scope'   => $this->acceptanceScope,
 		];
 	}
 }