fix(lint): restore noqa directives and register custom codes with ruff

Reverts the noqa: removal — the architecture validators (SVC-006,
SEC-034, MOD-004, API-007) use these to skip known-safe violations.
Added ruff lint.external config so ruff treats them as valid codes.

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

app/modules/inventory/services/inventory_service.py

@@ -754,7 +754,7 @@ class InventoryService:
         self, db: Session
     ) -> AdminStoresWithInventoryResponse:
         """Get list of stores that have inventory entries (admin only)."""
-        # SVC-005 - Admin function, intentionally cross-store
+        # noqa: SVC-005 - Admin function, intentionally cross-store
         # Use subquery to avoid DISTINCT on JSON columns (PostgreSQL can't compare JSON)
         store_ids_subquery = (
             db.query(Inventory.store_id)