prewarm_thread_message_counts

Prewarm thread selector cache for faster /threads open.

Fetches a bounded list of recent threads and populates checkpoint-derived fields for currently visible columns into the in-memory cache. Intended to run in a background worker during app startup and again whenever the session database has changed (e.g. after a turn writes new checkpoints), so the selector's first paint is never missing a thread the user just created.

Re-running this is cheap: the per-thread message-count and initial-prompt caches are keyed on checkpoint freshness, so only threads whose latest checkpoint changed are read back from disk.