refactor: remove legacy /shop and /api/v1/shop dead code

After the storefront migration, no live routes mount under /api/v1/shop/.
Remove all dead code that detected/handled shop API requests: the
is_shop_api_request() method, the shop API dispatch branch in middleware,
the RequestContext.SHOP enum member (renamed to STOREFRONT), legacy path
prefixes in FrontendDetector, and all associated tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/middleware/test_context.py

@@ -32,7 +32,7 @@ class TestRequestContextEnumBackwardCompatibility:
         assert RequestContext.API.value == "api"
         assert RequestContext.ADMIN.value == "admin"
         assert RequestContext.STORE_DASHBOARD.value == "store"
-        assert RequestContext.SHOP.value == "shop"
+        assert RequestContext.STOREFRONT.value == "storefront"
         assert RequestContext.FALLBACK.value == "fallback"
 
     def test_request_context_types(self):
@@ -101,7 +101,7 @@ class TestGetRequestContextBackwardCompatibility:
         assert context == RequestContext.STORE_DASHBOARD
 
     def test_get_request_context_maps_storefront(self):
-        """Test get_request_context maps FrontendType.STOREFRONT to RequestContext.SHOP."""
+        """Test get_request_context maps FrontendType.STOREFRONT to RequestContext.STOREFRONT."""
         from app.modules.enums import FrontendType
 
         request = Mock(spec=Request)
@@ -113,7 +113,7 @@ class TestGetRequestContextBackwardCompatibility:
             warnings.simplefilter("ignore", DeprecationWarning)
             context = get_request_context(request)
 
-        assert context == RequestContext.SHOP
+        assert context == RequestContext.STOREFRONT
 
     def test_get_request_context_maps_platform_to_fallback(self):
         """Test get_request_context maps FrontendType.PLATFORM to RequestContext.FALLBACK."""