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/PolicyRepository.php

@@ -18,14 +18,33 @@ class PolicyRepository {
 				'title'              => $policy->title,
 				'slug'               => $policy->slug,
 				'current_version_id' => $policy->currentVersionId,
+				'acceptance_scope'   => $policy->acceptanceScope,
 				'created_at'         => current_time( 'mysql' ),
 			],
-			[ '%s', '%s', '%d', '%s' ]
+			[ '%s', '%s', '%d', '%s', '%s' ]
 		);
 
 		return $this->db->insert_id;
 	}
 
+	/**
+	 * Policies that must be accepted in the given gate — those scoped to it or to
+	 * `both`. Pass `Policy::SCOPE_SIGNUP` or `Policy::SCOPE_BOOKING`.
+	 *
+	 * @return list<Policy>
+	 */
+	public function findForScope( string $scope ): array {
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				"SELECT * FROM {$this->table} WHERE acceptance_scope = %s OR acceptance_scope = %s ORDER BY title ASC",
+				$scope,
+				Policy::SCOPE_BOTH
+			)
+		);
+
+		return array_map( Policy::fromRow( ... ), $rows ?? [] );
+	}
+
 	public function updateCurrentVersion( int $policyId, int $versionId ): bool {
 		return false !== $this->db->update(
 			$this->table,