Summary

• make startup plugin dependency reinstall locking work on Windows and POSIX
• keep ensure_dependencies_installed() best-effort by logging lock/import/install failures instead of raising
• add regression coverage for no-op empty deps, Windows msvcrt locking, lock failures, and dependency install failures

Validation

• cd backend && uv run pytest tests/test_plugin_store.py tests/test_plugin_loader.py tests/test_plugin_manifest.py tests/test_plugin_node.py tests/test_plugins_api.py -q (34 passed)
• cd backend && uv run ruff check app/services/plugin_store.py tests/test_plugin_store.py
• cd backend && uv run ruff format --check app/services/plugin_store.py tests/test_plugin_store.py
• ./check.sh (1733 tests, 1733 passed)

Review

• Initial code review found missing install-failure coverage; fixed and amended.
• Final review reported no Critical, Important, or Minor findings.