feat(loyalty): Phase 4.1 — T&C via CMS integration

Add support for linking a loyalty program's Terms & Conditions to a
CMS page, replacing the simple terms_text textarea with a scalable
content source that supports rich HTML, multi-language, and store
overrides.

- Migration loyalty_006: adds terms_cms_page_slug column to
  loyalty_programs (nullable, String 200).
- Model + schemas: new field on LoyaltyProgram, ProgramCreate,
  ProgramUpdate, ProgramResponse.
- Program form: new "CMS Page Slug" input field with hint text,
  placed above the legacy terms_text (now labeled as "fallback").
- Enrollment page: when terms_cms_page_slug is set, JS fetches the
  CMS page content via /storefront/cms/pages/{slug} and displays
  rendered HTML in the modal. Falls back to terms_text when no slug.
- i18n: 3 new keys in 4 locales (terms_cms_page, terms_cms_page_hint,
  terms_fallback_hint).

Legacy terms_text field preserved as fallback for existing programs.

342 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/loyalty/static/storefront/js/loyalty-enroll.js

@@ -27,10 +27,26 @@ function customerLoyaltyEnroll() {
         enrolledCard: null,
         error: null,
         showTerms: false,
+        termsHtml: null,
 
         async init() {
             loyaltyEnrollLog.info('Customer loyalty enroll initializing...');
             await this.loadProgram();
+            // Load CMS T&C content if a page slug is configured
+            if (this.program?.terms_cms_page_slug) {
+                this.loadTermsFromCms(this.program.terms_cms_page_slug);
+            }
+        },
+
+        async loadTermsFromCms(slug) {
+            try {
+                const response = await apiClient.get(`/storefront/cms/pages/${slug}`);
+                if (response?.content_html) {
+                    this.termsHtml = response.content_html;
+                }
+            } catch (e) {
+                loyaltyEnrollLog.warn('Could not load CMS T&C page:', e.message);
+            }
         },
 
         async loadProgram() {