Move runtimes to json journal store

[tobiasraabe]

Summary

• move runtime persistence from sqlite to a JSON snapshot plus append-only journal in .pytask/
• add generic JSONL journal helpers for reuse and add journal recovery/flush tests
• flush runtimes on normal build exit and update profile runtime collection

Testing

• uv run --group test pytest tests/test_profile.py tests/test_runtime_store.py