Add Policies domain (drafting, versioning, tracked acceptance)

Implements #6: studio admins draft, version, and publish policies; the
public registration gate reads the current published version of each, and
acceptance is recorded against the exact version so a new version must be
re-accepted at the next booking.

- src/Policy/: Policy, PolicyVersion, PolicyAcceptance value objects;
  PolicyRepository, PolicyVersionRepository, AcceptanceRepository;
  PolicyService (orchestrates create/add-draft/publish across the policies
  and versions tables); PolicyEndpoint (REST); PolicyController +
  templates/admin/policies.php (Policies admin menu, manage_policies)
- us_policies, us_policy_versions, us_policy_acceptances tables in Schema
- REST: public GET /policies (current published versions); manage_policies
  for create, add version, edit draft, and publish
- Wiring in Plugin, RestRegistrar, AdminMenu

AcceptanceRepository is built now and consumed by the booking/enrolment
gate in #3/#4.

Also bump PHPStan to --memory-limit=1G in the composer lint script; the
default 128M now crashes the analysis as the codebase has grown.

Tests: tests/Unit/Policy/ (value objects, repositories, service).
composer test (90 total), cs, and PHPStan level 6 all pass.

Refs #6

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

src/Policy/Policy.php

@@ -0,0 +1,37 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Policy;
+
+class Policy {
+
+	public function __construct(
+		public readonly string $title,
+		public readonly string $slug,
+		public readonly ?int $currentVersionId = null,
+		public readonly ?int $id = null,
+	) {}
+
+	public static function fromRow( object $row ): self {
+		return new self(
+			title:            $row->title,
+			slug:             $row->slug,
+			currentVersionId: null !== $row->current_version_id ? (int) $row->current_version_id : null,
+			id:               (int) $row->id,
+		);
+	}
+
+	/**
+	 * Returns a plain array representation of the policy.
+	 *
+	 * @return array<string, mixed>
+	 */
+	public function toArray(): array {
+		return [
+			'id'                 => $this->id,
+			'title'              => $this->title,
+			'slug'               => $this->slug,
+			'current_version_id' => $this->currentVersionId,
+		];
+	}
+}