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/models/loyalty_program.py

@@ -227,7 +227,12 @@ class LoyaltyProgram(Base, TimestampMixin, SoftDeleteMixin):
     terms_text = Column(
         Text,
         nullable=True,
-        comment="Loyalty program terms and conditions",
+        comment="Loyalty program terms and conditions (legacy — use terms_cms_page_slug when available)",
+    )
+    terms_cms_page_slug = Column(
+        String(200),
+        nullable=True,
+        comment="CMS page slug for full T&C content (overrides terms_text when set)",
     )
     privacy_url = Column(
         String(500),