refactor: centralize frontend detection with FrontendDetector

Major architecture change to unify frontend detection:

## Problem Solved
- Eliminated code duplication across 3 middleware files
- Fixed incomplete path detection (now detects /api/v1/admin/*)
- Unified on FrontendType enum (deprecates RequestContext)
- Added request.state.frontend_type for all requests

## New Components
- app/core/frontend_detector.py: Centralized FrontendDetector class
- middleware/frontend_type.py: FrontendTypeMiddleware (replaces ContextMiddleware)
- docs/architecture/frontend-detection.md: Complete architecture documentation

## Changes
- main.py: Use FrontendTypeMiddleware instead of ContextMiddleware
- middleware/context.py: Deprecated (kept for backwards compatibility)
- middleware/platform_context.py: Use FrontendDetector.is_admin()
- middleware/vendor_context.py: Use FrontendDetector.is_admin()
- middleware/language.py: Use FrontendType instead of context_value
- app/exceptions/handler.py: Use FrontendType.STOREFRONT
- app/exceptions/error_renderer.py: Use FrontendType
- Customer routes: Cookie path changed from /shop to /storefront

## Documentation
- docs/architecture/frontend-detection.md: New comprehensive docs
- docs/architecture/middleware.md: Updated for new system
- docs/architecture/request-flow.md: Updated for FrontendType
- docs/backend/middleware-reference.md: Updated API reference

## Tests
- tests/unit/core/test_frontend_detector.py: 37 new tests
- tests/unit/middleware/test_frontend_type.py: 11 new tests
- tests/unit/middleware/test_context.py: Updated for compatibility

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

middleware/vendor_context.py

@@ -23,6 +23,7 @@ from starlette.middleware.base import BaseHTTPMiddleware
 
 from app.core.config import settings
 from app.core.database import get_db
+from app.core.frontend_detector import FrontendDetector
 from app.modules.tenancy.models import Vendor
 from app.modules.tenancy.models import VendorDomain
 
@@ -194,22 +195,20 @@ class VendorContextManager:
 
     @staticmethod
     def is_admin_request(request: Request) -> bool:
-        """Check if request is for admin interface."""
+        """
+        Check if request is for admin interface.
+
+        DEPRECATED: Use FrontendDetector.is_admin() instead.
+        Kept for backwards compatibility.
+        """
         host = request.headers.get("host", "")
         path = request.url.path
-
-        if ":" in host:
-            host = host.split(":")[0]
-
-        if host.startswith("admin."):
-            return True
-
-        return path.startswith("/admin")
+        return FrontendDetector.is_admin(host, path)
 
     @staticmethod
     def is_api_request(request: Request) -> bool:
         """Check if request is for API endpoints."""
-        return request.url.path.startswith("/api/")
+        return FrontendDetector.is_api_request(request.url.path)
 
     @staticmethod
     def is_shop_api_request(request: Request) -> bool: