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/schemas/content_page.py

@@ -90,7 +90,6 @@ class ContentPageResponse(BaseModel):
     show_in_header: bool
     show_in_legal: bool
     is_platform_page: bool = False
-    is_platform_default: bool = False  # Deprecated: use is_platform_page
     is_store_default: bool = False
     is_store_override: bool = False
     page_tier: str | None = None