Without a hint SQLite answers the correlated NOT EXISTS with a per-thread
stream index scan. On 3000 imported threads with 30000 events that took
150-270ms here and 20.7s in review; with the partial index the importer
already names, it takes about 1.5ms. The index ships in the same migration
as the V2 tables the guard checks for.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>