refactor: rename shop to storefront throughout codebase

Rename "shop" to "storefront" as not all platforms sell items -
storefront is a more accurate term for the customer-facing interface.

Changes:
- Rename app/api/v1/shop/ → app/api/v1/storefront/
- Rename app/routes/shop_pages.py → app/routes/storefront_pages.py
- Rename app/modules/cms/routes/api/shop.py → storefront.py
- Rename tests/integration/api/v1/shop/ → storefront/
- Update API prefix from /api/v1/shop to /api/v1/storefront
- Update route tags from shop-* to storefront-*
- Rename get_shop_context() → get_storefront_context()
- Update architecture rules to reference storefront paths
- Update all test API endpoint paths

This is Phase 2 of the storefront module restructure plan.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.architecture-rules/api.yaml

@@ -201,7 +201,7 @@ api_endpoint_rules:
       Use request.state.vendor_id from middleware.
     pattern:
       file_pattern: "app/api/v1/vendor/**/*.py"
-      file_pattern: "app/api/v1/shop/**/*.py"
+      file_pattern: "app/api/v1/storefront/**/*.py"
       discouraged_patterns:
         - "db.query(.*).all()"