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/messaging/schemas/__init__.py

@@ -34,7 +34,6 @@ from app.modules.messaging.schemas.message import (
     ConversationCreate,
     ConversationDetailResponse,
     ConversationListResponse,
-    ConversationResponse,
     ConversationSummary,
     MarkReadResponse,
     # Message schemas
@@ -90,7 +89,6 @@ __all__ = [
     "ConversationSummary",
     "ConversationDetailResponse",
     "ConversationListResponse",
-    "ConversationResponse",
     # Unread count
     "UnreadCountResponse",
     # Notification preferences

app/modules/messaging/schemas/message.py

@@ -192,10 +192,6 @@ class ConversationListResponse(BaseModel):
     limit: int
 
 
-# Backward compatibility alias
-ConversationResponse = ConversationDetailResponse
-
-
 # ============================================================================
 # Unread Count Schemas
 # ============================================================================