Add fee forecast benchmark export pipeline

[Bortlesboat]

Summary

• add a local benchmark export pipeline that writes importer-compatible JSONL from fee history and confirmed block outcomes
• make the background fee collector populate the research tables needed by that export during normal API operation
• harden test fixtures so request tests do not start background jobs and create flaky fee-history assertions

Test Plan

• PYTHONPATH=src python -m pytest tests/test_jobs.py tests/test_fee_benchmark_export.py tests/test_fees.py -q
• PYTHONPATH=src python -m pytest tests/test_misc.py -q -k migration