refactor: remove backward compatibility code for pre-launch baseline

Clean up accumulated backward-compat shims, deprecated wrappers, unused
aliases, and legacy code across the codebase. Since the platform is not
live yet, this establishes a clean baseline.

Changes:
- Delete deprecated middleware/context.py (RequestContext, get_request_context)
- Remove unused factory get_store_email_settings_service()
- Remove deprecated pagination_full macro, /admin/platform-homepage route
- Remove ConversationResponse, InvoiceSettings* unprefixed aliases
- Simplify celery_config.py (remove empty LEGACY_TASK_MODULES)
- Standardize billing exceptions: *Error aliases → *Exception names
- Consolidate duplicate TierNotFoundError/FeatureNotFoundError classes
- Remove deprecated is_admin_request() from Store/PlatformContextManager
- Remove is_platform_default field, MediaUploadResponse legacy flat fields
- Remove MediaItemResponse.url alias, update JS to use file_url
- Update all affected tests and documentation

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

middleware/store_context.py

@@ -222,18 +222,6 @@ class StoreContextManager:
 
         return request.url.path
 
-    @staticmethod
-    def is_admin_request(request: Request) -> bool:
-        """
-        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
-        return FrontendDetector.is_admin(host, path)
-
     @staticmethod
     def is_api_request(request: Request) -> bool:
         """Check if request is for API endpoints."""
@@ -430,7 +418,7 @@ class StoreContextMiddleware(BaseHTTPMiddleware):
         """
         # Skip store detection for admin, static files, and system requests
         if (
-            StoreContextManager.is_admin_request(request)
+            FrontendDetector.is_admin(request.headers.get("host", ""), request.url.path)
             or StoreContextManager.is_static_file_request(request)
             or request.url.path in ["/", "/health", "/docs", "/redoc", "/openapi.json"]
         ):