refactor(B1): migrate to domain-driven project structure

Move all models/schemas/services/routers into app/domains/.
Keep backward-compat shims in old locations for imports.
Preserves domains/rendering/tasks.py from Phase A.

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

backend/app/domains/imports/service.py

@@ -0,0 +1,12 @@
+"""Import services — Excel parsing and product import."""
+
+# Re-export from original service files for backward compatibility.
+from app.services.excel_parser import parse_excel, parsed_excel_to_dict
+from app.services.excel_import import import_excel_to_products, preview_excel_rows
+
+__all__ = [
+    "parse_excel",
+    "parsed_excel_to_dict",
+    "import_excel_to_products",
+    "preview_excel_rows",
+]