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/core/test_frontend_detector.py

@@ -7,7 +7,6 @@ Tests cover:
 - Path-based detection (dev mode)
 - Subdomain-based detection (prod mode)
 - Custom domain detection
-- Legacy /shop/ path support
 - Priority order of detection methods
 """
 
@@ -103,15 +102,6 @@ class TestFrontendDetectorStorefront:
         )
         assert result == FrontendType.STOREFRONT
 
-    def test_detect_storefront_legacy_shop_path(self):
-        """Test storefront detection from legacy /shop path."""
-        result = FrontendDetector.detect(host="localhost", path="/shop/products")
-        assert result == FrontendType.STOREFRONT
-
-    def test_detect_storefront_legacy_shop_api_path(self):
-        """Test storefront detection from legacy /api/v1/shop path."""
-        result = FrontendDetector.detect(host="localhost", path="/api/v1/shop/cart")
-        assert result == FrontendType.STOREFRONT
 
 
 @pytest.mark.unit