Add export/import org scope regression tests

[jackmusick]

Summary

• add E2E import coverage for exact-scope configs, tables, and integration mappings
• preserve global rows when importing org-scoped configs with the same key
• add a concrete global target sentinel for multipart import forms and cover it in unit tests
• verify integration mapping imports update the exact org row without dropping existing OAuth token links

Tests

• python3 -m py_compile api/src/routers/export_import.py api/tests/e2e/api/test_export_import_scope.py api/tests/unit/routers/test_export_import.py
• cd api && ruff check src/routers/export_import.py tests/e2e/api/test_export_import_scope.py tests/unit/routers/test_export_import.py
• ./test.sh tests/unit/routers/test_export_import.py -v
• ./test.sh tests/e2e/api/test_export_import_scope.py -v

Refs #313