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/locales/en.json

@@ -405,7 +405,10 @@
       "logo_url_help": "Required for Google Wallet integration. Must be a publicly accessible image URL (PNG or JPG).",
       "hero_image_url": "Hero Image URL",
       "terms_privacy": "Terms & Privacy",
-      "terms_conditions": "Terms & Conditions",
+      "terms_cms_page": "CMS Page Slug",
+      "terms_cms_page_hint": "Enter a CMS page slug (e.g. terms-and-conditions) to display full T&C from the CMS module",
+      "terms_conditions": "Terms & Conditions (fallback)",
+      "terms_fallback_hint": "Used when no CMS page slug is set",
       "privacy_policy_url": "Privacy Policy URL",
       "program_status": "Program Status",
       "program_active": "Program Active",