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/PolicyService.php

@@ -0,0 +1,58 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Policy;
+
+/**
+ * Orchestrates policy operations that span the policies and versions tables.
+ */
+class PolicyService {
+
+	public function __construct(
+		private PolicyRepository $policies,
+		private PolicyVersionRepository $versions,
+	) {}
+
+	public function createPolicy( string $title, string $slug ): int {
+		return $this->policies->insert( new Policy( $title, $slug ) );
+	}
+
+	/**
+	 * Add a new draft version to a policy, numbered after the latest existing one.
+	 */
+	public function addDraftVersion( int $policyId, ?string $body ): int {
+		$next = $this->versions->maxVersionNumber( $policyId ) + 1;
+
+		return $this->versions->insert(
+			new PolicyVersion(
+				policyId:      $policyId,
+				versionNumber: $next,
+				body:          $body,
+				status:        PolicyVersion::STATUS_DRAFT,
+			)
+		);
+	}
+
+	/**
+	 * Publish a version: archive the policy's previously current version, mark
+	 * the new one published, and point the policy at it. Returns false if the
+	 * version does not belong to the policy.
+	 */
+	public function publishVersion( int $policyId, int $versionId ): bool {
+		$policy  = $this->policies->findById( $policyId );
+		$version = $this->versions->findById( $versionId );
+
+		if ( null === $policy || null === $version || $version->policyId !== $policyId ) {
+			return false;
+		}
+
+		if ( null !== $policy->currentVersionId && $policy->currentVersionId !== $versionId ) {
+			$this->versions->updateStatus( $policy->currentVersionId, PolicyVersion::STATUS_ARCHIVED );
+		}
+
+		$this->versions->updateStatus( $versionId, PolicyVersion::STATUS_PUBLISHED, current_time( 'mysql' ) );
+		$this->policies->updateCurrentVersion( $policyId, $versionId );
+
+		return true;
+	}
+}