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/routes.py

@@ -61,7 +61,7 @@ class RouteInfo:
     include_in_schema: bool = True
     module_code: str = ""
     route_type: str = ""  # "api" or "pages"
-    frontend: str = ""  # "admin", "store", "shop"
+    frontend: str = ""  # "admin", "store", "storefront"
     priority: int = 0  # Higher = registered later (for catch-all routes)
     custom_prefix: str = ""  # Custom prefix from ROUTE_CONFIG
 
@@ -76,7 +76,7 @@ def discover_module_routes() -> list[RouteInfo]:
     Route discovery looks for:
     - routes/api/admin.py -> admin API routes
     - routes/api/store.py -> store API routes
-    - routes/api/shop.py -> shop API routes
+    - routes/api/storefront.py -> storefront API routes
     - routes/pages/admin.py -> admin page routes
     - routes/pages/store.py -> store page routes