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/core/celery_config.py

@@ -9,13 +9,6 @@ It includes:
 - Task retry policies
 - Sentry integration for error tracking
 - Module-based task discovery (discovers tasks from app/modules/*/tasks/)
-
-Task Discovery:
-- Legacy tasks: Explicitly listed in the 'include' parameter
-- Module tasks: Auto-discovered via discover_module_tasks()
-
-As modules are migrated, their tasks will move from the legacy include list
-to automatic discovery from the module's tasks/ directory.
 """
 
 import logging
@@ -48,38 +41,27 @@ if SENTRY_DSN:
 # =============================================================================
 # TASK DISCOVERY
 # =============================================================================
-# Legacy tasks (will be migrated to modules over time)
-# MIGRATION STATUS:
-# - subscription: MIGRATED to billing module (kept for capture_capacity_snapshot -> monitoring)
-# - marketplace, letzshop, export: MIGRATED to marketplace module
-# - code_quality, test_runner: Will migrate to dev-tools module
-LEGACY_TASK_MODULES: list[str] = [
-    # All legacy tasks have been migrated to their respective modules.
-    # Task discovery now happens via app.modules.tasks.discover_module_tasks()
-]
 
 
 def get_all_task_modules() -> list[str]:
     """
-    Get all task modules (legacy + module-based).
+    Get all task modules via module-based discovery.
 
     Returns:
-        Combined list of legacy task modules and discovered module tasks
+        List of discovered module task packages
     """
-    all_modules = list(LEGACY_TASK_MODULES)
-
     try:
         from app.modules.tasks import discover_module_tasks
 
         module_tasks = discover_module_tasks()
-        all_modules.extend(module_tasks)
         logger.info(f"Discovered {len(module_tasks)} module task packages")
+        return module_tasks
     except ImportError as e:
         logger.warning(f"Could not import module task discovery: {e}")
     except Exception as e:
         logger.error(f"Error discovering module tasks: {e}")
 
-    return all_modules
+    return []
 
 
 # Create Celery application