Fix parallel inventory calculation with dirty Brightway databases

[langeluca]

Summary

• clean pending Brightway database changes in the parent process before starting parallel inventory workers
• prevent spawned workers from concurrently writing the shared Brightway database metadata file
• add a regression test that verifies the registry is cleaned before the process executor is constructed

Fixes #68.

Validation

• uv run --extra dev python -m pytest tests/test_background_inventory_performance.py::test_parallel_cleans_database_registry_before_workers -q
• Black, isort, and Flake8 pass for the changed test; Black and isort pass for both changed files
• the original Windows notebook reproduction completed parallel inventory calculation for four background databases after applying the same parent-process clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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