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>

app/core/frontend_detector.py

@@ -46,8 +46,6 @@ class FrontendDetector:
     STOREFRONT_PATH_PREFIXES = (
         "/storefront",
         "/api/v1/storefront",
-        "/shop",  # Legacy support
-        "/api/v1/shop",  # Legacy support
         "/stores/",  # Path-based store access
     )
     MERCHANT_PATH_PREFIXES = ("/merchants", "/api/v1/merchants")
@@ -113,7 +111,7 @@ class FrontendDetector:
             return FrontendType.PLATFORM
 
         # 3. Store subdomain detection (wizamart.oms.lu)
-        # If subdomain exists and is not reserved -> it's a store shop
+        # If subdomain exists and is not reserved -> it's a store storefront
         if subdomain and subdomain not in cls.RESERVED_SUBDOMAINS:
             logger.debug(
                 f"[FRONTEND_DETECTOR] Detected STOREFRONT from subdomain: {subdomain}"