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/platform_context.py

@@ -257,18 +257,6 @@ class PlatformContextManager:
         # For domain-based or default, path remains unchanged
         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_static_file_request(request: Request) -> bool:
         """Check if request is for static files."""
@@ -478,16 +466,6 @@ class PlatformContextMiddleware:
             return True
         return "favicon.ico" in path_lower
 
-    def _is_admin_request(self, path: str, host: str) -> bool:
-        """
-        Check if request is for admin interface.
-
-        DEPRECATED: Use FrontendDetector.is_admin() instead.
-        Kept for backwards compatibility.
-        """
-        return FrontendDetector.is_admin(host, path)
-
-
 def get_current_platform(request: Request) -> Platform | None:
     """Helper function to get current platform from request state."""
     return getattr(request.state, "platform", None)