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/Schema.php

@@ -89,6 +89,43 @@ class Schema {
                 KEY registration (registration_type, registration_id),
                 KEY student_id (student_id)
             ) {$charset};",
+
+			"CREATE TABLE {$prefix}us_policies (
+                id                 BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                title              VARCHAR(191)    NOT NULL,
+                slug               VARCHAR(191)    NOT NULL,
+                current_version_id BIGINT UNSIGNED DEFAULT NULL,
+                created_at         DATETIME        NOT NULL,
+                PRIMARY KEY (id),
+                UNIQUE KEY slug (slug)
+            ) {$charset};",
+
+			"CREATE TABLE {$prefix}us_policy_versions (
+                id             BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                policy_id      BIGINT UNSIGNED NOT NULL,
+                version_number INT             NOT NULL DEFAULT 1,
+                body           LONGTEXT,
+                status         VARCHAR(20)     NOT NULL DEFAULT 'draft',
+                published_at   DATETIME        DEFAULT NULL,
+                created_at     DATETIME        NOT NULL,
+                PRIMARY KEY (id),
+                KEY policy_id (policy_id),
+                KEY status (status)
+            ) {$charset};",
+
+			"CREATE TABLE {$prefix}us_policy_acceptances (
+                id                BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                policy_version_id BIGINT UNSIGNED NOT NULL,
+                student_id        BIGINT UNSIGNED NOT NULL,
+                registration_type VARCHAR(20)     NOT NULL,
+                registration_id   BIGINT UNSIGNED NOT NULL,
+                accepted_at       DATETIME        NOT NULL,
+                ip_address        VARCHAR(45)     DEFAULT NULL,
+                PRIMARY KEY (id),
+                KEY policy_version_id (policy_version_id),
+                KEY student_id (student_id),
+                KEY registration (registration_type, registration_id)
+            ) {$charset};",
 		];
 	}
 }