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>

tests/Unit/Policy/PolicyRepositoryTest.php

@@ -0,0 +1,94 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Policy;
+
+use Brain\Monkey\Functions;
+use Mockery;
+use Unsupervised\Schedular\Policy\Policy;
+use Unsupervised\Schedular\Policy\PolicyRepository;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class PolicyRepositoryTest extends TestCase
+{
+    private \wpdb $db;
+    private PolicyRepository $repo;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->db         = Mockery::mock(\wpdb::class);
+        $this->db->prefix = 'wp_';
+        $this->repo       = new PolicyRepository($this->db);
+    }
+
+    public function testInsertReturnsId(): void
+    {
+        Functions\expect('current_time')->with('mysql')->andReturn('2026-06-01 12:00:00');
+
+        $this->db->shouldReceive('insert')
+            ->once()
+            ->with(
+                'wp_us_policies',
+                Mockery::on(static fn (array $d): bool => $d['title'] === 'Cancellation' && $d['slug'] === 'cancellation' && $d['current_version_id'] === null),
+                ['%s', '%s', '%d', '%s']
+            );
+        $this->db->insert_id = 7;
+
+        self::assertSame(7, $this->repo->insert(new Policy('Cancellation', 'cancellation')));
+    }
+
+    public function testUpdateCurrentVersion(): void
+    {
+        $this->db->shouldReceive('update')
+            ->once()
+            ->with('wp_us_policies', ['current_version_id' => 9], ['id' => 7], ['%d'], ['%d'])
+            ->andReturn(1);
+
+        self::assertTrue($this->repo->updateCurrentVersion(7, 9));
+    }
+
+    public function testFindBySlugReturnsPolicy(): void
+    {
+        $this->db->shouldReceive('prepare')->andReturn('SELECT ...');
+        $this->db->shouldReceive('get_row')->andReturn((object) [
+            'id'                 => '7',
+            'title'              => 'Cancellation',
+            'slug'               => 'cancellation',
+            'current_version_id' => null,
+        ]);
+
+        $policy = $this->repo->findBySlug('cancellation');
+
+        self::assertInstanceOf(Policy::class, $policy);
+        self::assertSame('cancellation', $policy->slug);
+    }
+
+    public function testFindBySlugReturnsNullWhenMissing(): void
+    {
+        $this->db->shouldReceive('prepare')->andReturn('SELECT ...');
+        $this->db->shouldReceive('get_row')->andReturn(null);
+
+        self::assertNull($this->repo->findBySlug('nope'));
+    }
+
+    public function testFindAllMapsRows(): void
+    {
+        $this->db->shouldReceive('get_results')->andReturn([
+            (object) ['id' => '1', 'title' => 'A', 'slug' => 'a', 'current_version_id' => null],
+        ]);
+
+        $all = $this->repo->findAll();
+
+        self::assertCount(1, $all);
+        self::assertInstanceOf(Policy::class, $all[0]);
+    }
+
+    public function testDeleteCallsWpdb(): void
+    {
+        $this->db->shouldReceive('delete')->once()->with('wp_us_policies', ['id' => 3], ['%d'])->andReturn(1);
+
+        self::assertTrue($this->repo->delete(3));
+    }
+}