Batch DB commits during score/tailor/cover instead of one commit at the end

[msohailse]

Summary

score, tailor, and cover-letter stages each collected every result in memory and only wrote to the DB in a single commit after the whole job list finished. If the run got interrupted partway through -- rate limit, network drop, killed process, crash -- nothing already completed that run had been persisted, so it was all lost.

• All three stages now flush to the DB every 5 jobs (and on the final job), instead of one commit at the very end.
• Pure DB-durability fix, no provider/LLM-specific changes -- applies cleanly on top of current main.

Test plan

• ruff check / ruff format --check on all three files -- identical finding count to unmodified main (18 pre-existing, 0 new)
• Verified behavior: killed a run mid-batch and confirmed only the in-progress batch (up to 5 jobs) was lost, not the whole run's prior progress

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