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/billing/services/__init__.py

@@ -5,14 +5,6 @@ Billing module services.
 Provides subscription management, Stripe integration, and admin operations.
 """
 
-from app.modules.billing.exceptions import (
-    BillingServiceError,
-    NoActiveSubscriptionError,
-    PaymentSystemNotConfiguredError,
-    StripePriceNotConfiguredError,
-    SubscriptionNotCancelledError,
-    TierNotFoundError,
-)
 from app.modules.billing.services.admin_subscription_service import (
     AdminSubscriptionService,
     admin_subscription_service,
@@ -56,12 +48,6 @@ __all__ = [
     "admin_subscription_service",
     "BillingService",
     "billing_service",
-    "BillingServiceError",
-    "PaymentSystemNotConfiguredError",
-    "TierNotFoundError",
-    "StripePriceNotConfiguredError",
-    "NoActiveSubscriptionError",
-    "SubscriptionNotCancelledError",
     "FeatureService",
     "feature_service",
     "PlatformPricingService",