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/orders/schemas/__init__.py

@@ -21,10 +21,6 @@ from app.modules.orders.schemas.invoice import (
     InvoiceResponse,
     # Address schemas
     InvoiceSellerDetails,
-    # Backward compatibility
-    InvoiceSettingsCreate,
-    InvoiceSettingsResponse,
-    InvoiceSettingsUpdate,
     InvoiceStatsResponse,
     InvoiceStatusUpdate,
     # Invoice settings schemas
@@ -148,8 +144,4 @@ __all__ = [
     # PDF
     "InvoicePDFGeneratedResponse",
     "InvoiceStatsResponse",
-    # Backward compatibility
-    "InvoiceSettingsCreate",
-    "InvoiceSettingsUpdate",
-    "InvoiceSettingsResponse",
 ]

app/modules/orders/schemas/invoice.py

@@ -308,9 +308,3 @@ class InvoiceStatsResponse(BaseModel):
     @property
     def total_revenue(self) -> float:
         return self.total_revenue_cents / 100
-
-
-# Backward compatibility re-exports
-InvoiceSettingsCreate = StoreInvoiceSettingsCreate
-InvoiceSettingsUpdate = StoreInvoiceSettingsUpdate
-InvoiceSettingsResponse = StoreInvoiceSettingsResponse