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>

docs/features/subscription-billing.md

@@ -582,11 +582,11 @@ Custom exceptions for billing operations (`app/modules/billing/exceptions.py`):
 
 | Exception | HTTP Status | Description |
 |-----------|-------------|-------------|
-| `PaymentSystemNotConfiguredError` | 503 | Stripe not configured |
-| `TierNotFoundError` | 404 | Invalid tier code |
-| `StripePriceNotConfiguredError` | 400 | No Stripe price for tier |
-| `NoActiveSubscriptionError` | 400 | Operation requires subscription |
-| `SubscriptionNotCancelledError` | 400 | Cannot reactivate active subscription |
+| `PaymentSystemNotConfiguredException` | 503 | Stripe not configured |
+| `TierNotFoundException` | 404 | Invalid tier code |
+| `StripePriceNotConfiguredException` | 400 | No Stripe price for tier |
+| `NoActiveSubscriptionException` | 400 | Operation requires subscription |
+| `SubscriptionNotCancelledException` | 400 | Cannot reactivate active subscription |
 
 ## Testing