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/migrations/versions/loyalty_006_add_terms_cms_page_slug.py

@@ -0,0 +1,35 @@
+"""loyalty 006 - add terms_cms_page_slug to loyalty_programs
+
+Allows linking a loyalty program's T&C to a CMS page instead of
+using the simple terms_text field. When set, the enrollment page
+resolves the slug to a full CMS page. The legacy terms_text is
+kept as a fallback for existing programs.
+
+Revision ID: loyalty_006
+Revises: loyalty_005
+Create Date: 2026-04-11
+"""
+import sqlalchemy as sa
+
+from alembic import op
+
+revision = "loyalty_006"
+down_revision = "loyalty_005"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "loyalty_programs",
+        sa.Column(
+            "terms_cms_page_slug",
+            sa.String(200),
+            nullable=True,
+            comment="CMS page slug for full T&C content (overrides terms_text when set)",
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("loyalty_programs", "terms_cms_page_slug")