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>

middleware/context.py

@@ -14,7 +14,7 @@ Migration guide:
 - RequestContext.API -> Check with FrontendDetector.is_api_request()
 - RequestContext.ADMIN -> FrontendType.ADMIN
 - RequestContext.STORE_DASHBOARD -> FrontendType.STORE
-- RequestContext.SHOP -> FrontendType.STOREFRONT
+- RequestContext.STOREFRONT -> FrontendType.STOREFRONT
 - RequestContext.FALLBACK -> FrontendType.PLATFORM (or handle API separately)
 
 - get_request_context(request) -> get_frontend_type(request)
@@ -44,14 +44,14 @@ class RequestContext(str, Enum):
     - API -> Use FrontendDetector.is_api_request() + FrontendType
     - ADMIN -> FrontendType.ADMIN
     - STORE_DASHBOARD -> FrontendType.STORE
-    - SHOP -> FrontendType.STOREFRONT
+    - STOREFRONT -> FrontendType.STOREFRONT
     - FALLBACK -> FrontendType.PLATFORM
     """
 
     API = "api"
     ADMIN = "admin"
     STORE_DASHBOARD = "store"
-    SHOP = "shop"
+    STOREFRONT = "storefront"
     FALLBACK = "fallback"
 
 
@@ -82,7 +82,7 @@ def get_request_context(request: Request) -> RequestContext:
     mapping = {
         FrontendType.ADMIN: RequestContext.ADMIN,
         FrontendType.STORE: RequestContext.STORE_DASHBOARD,
-        FrontendType.STOREFRONT: RequestContext.SHOP,
+        FrontendType.STOREFRONT: RequestContext.STOREFRONT,
         FrontendType.PLATFORM: RequestContext.FALLBACK,
     }