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>

app/modules/cms/services/__init__.py

@@ -15,7 +15,6 @@ from app.modules.cms.services.media_service import (
 )
 from app.modules.cms.services.store_email_settings_service import (
     StoreEmailSettingsService,
-    get_store_email_settings_service,  # Deprecated: use store_email_settings_service
     store_email_settings_service,
 )
 from app.modules.cms.services.store_theme_service import (
@@ -32,5 +31,4 @@ __all__ = [
     "store_theme_service",
     "StoreEmailSettingsService",
     "store_email_settings_service",
-    "get_store_email_settings_service",  # Deprecated
 ]