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/routes/api/store_features.py

@@ -24,7 +24,7 @@ from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_store_api, require_module_access
 from app.core.database import get_db
-from app.modules.billing.exceptions import FeatureNotFoundError
+from app.modules.billing.exceptions import FeatureNotFoundException
 from app.modules.billing.schemas.billing import (
     CategoryListResponse,
     FeatureCodeListResponse,
@@ -275,7 +275,7 @@ def get_feature_detail(
     # Get feature declaration
     decl = feature_aggregator.get_declaration(feature_code)
     if not decl:
-        raise FeatureNotFoundError(feature_code)
+        raise FeatureNotFoundException(feature_code)
 
     # Check availability
     is_available = feature_service.has_feature(db, merchant_id, platform_id, feature_code)