Clear Research History

Delete the caller's saved Ask questions and answers and price-move explanations.

Ask answers are also copied into research_result_thesis_impacts (artifact_id is the ask_history row), so those rows go too; otherwise the answers outlive the history. Price-move impacts hang off alerts, not this history, and stay. A completed conversational Ask receipt must point at its answer, so the ON DELETE SET NULL on thesis_ask_command_confirmations would fail its check: those receipts go first. Account authority only: every non-safe /users route is, so a script key cannot clear it.