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>

composer.json

@@ -29,7 +29,7 @@
     "scripts": {
         "test": "phpunit --configuration phpunit.xml",
         "test:coverage": "phpunit --configuration phpunit.xml --coverage-html coverage/",
-        "lint": "phpstan analyse src/ --level=6 --configuration phpstan.neon",
+        "lint": "phpstan analyse src/ --level=6 --configuration phpstan.neon --memory-limit=1G",
         "cs": "phpcs --standard=phpcs.xml.dist",
         "cs:fix": "phpcbf --standard=phpcs.xml.dist",
         "build": "bash bin/build-zip.sh"