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>

docs/features/policies.md

@@ -11,6 +11,7 @@ The studio admin drafts, versions, and publishes policies (e.g. cancellation, pa
 | `title`              | VARCHAR(191)     | Display name                                   |
 | `slug`               | VARCHAR(191)     | Unique key, e.g. `cancellation`                |
 | `current_version_id` | BIGINT UNSIGNED  | Nullable FK → `us_policy_versions.id` (published) |
+| `acceptance_scope`   | VARCHAR(20)      | `signup` / `booking` / `both` — when it must be accepted |
 | `created_at`         | DATETIME         | Insertion time                                 |
 
 ## Data Model — `{prefix}us_policy_versions`