refactor: remove all backward compatibility code across 70 files

Clean up 28 backward compatibility instances identified in the codebase.
The app is not live, so all shims are replaced with the target architecture:

- Remove legacy Inventory.location column (use bin_location exclusively)
- Remove dashboard _extract_metric_value helper (use flat metrics dict)
- Remove legacy stat field duplicates (total_stores, total_imports, etc.)
- Remove 13 re-export shims and class aliases across modules
- Remove module-enabling JSON fallback (use PlatformModule junction table)
- Remove menu_to_legacy_format() conversion (return dataclasses directly)
- Remove title/description from MarketplaceProductBase schema
- Clean billing convenience method docstrings
- Clean test fixtures and backward-compat comments
- Add PlatformModule seeding to init_production.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/cms/routes/__init__.py

@@ -6,9 +6,9 @@ This module provides functions to register CMS routes
 with module-based access control.
 
 NOTE: Routers are NOT auto-imported to avoid circular dependencies.
-Import directly from admin.py or store.py as needed:
-  from app.modules.cms.routes.admin import admin_router
-  from app.modules.cms.routes.store import store_router, store_media_router
+Import directly from api/admin.py or api/store.py as needed:
+  from app.modules.cms.routes.api.admin import admin_router
+  from app.modules.cms.routes.api.store import store_router
 """
 
 # Routers are imported on-demand to avoid circular dependencies
@@ -20,12 +20,12 @@ __all__ = ["admin_router", "store_router", "store_media_router"]
 def __getattr__(name: str):
     """Lazy import routers to avoid circular dependencies."""
     if name == "admin_router":
-        from app.modules.cms.routes.admin import admin_router
+        from app.modules.cms.routes.api.admin import admin_router
         return admin_router
     if name == "store_router":
-        from app.modules.cms.routes.store import store_router
+        from app.modules.cms.routes.api.store import store_router
         return store_router
     if name == "store_media_router":
-        from app.modules.cms.routes.store import store_media_router
+        from app.modules.cms.routes.api.store_media import store_media_router
         return store_media_router
     raise AttributeError(f"module {__name__!r} has no attribute {name!r}")