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>

tests/integration/middleware/middleware_test_routes.py

@@ -377,19 +377,6 @@ async def test_clean_path_context(request: Request):
     }
 
 
-@api_router.get("/enum")
-async def test_api_enum(request: Request):
-    """Test context enum instance."""
-    from middleware.context import RequestContext
-
-    context = getattr(request.state, "context_type", None)
-    return {
-        "is_enum": isinstance(context, RequestContext) if context else False,
-        "enum_name": context.name if context else None,
-        "enum_value": context.value if context else None,
-    }
-
-
 @api_router.get("/theme")
 async def test_api_theme(request: Request):
     """Test theme in API context."""