Deep mode fires many create_asset calls at once. UpsertAssets used a
DEFERRED transaction (SELECT then INSERT) on a 25-connection pool, so
writers hit SQLITE_BUSY immediately and returned "database is locked"
in a few milliseconds instead of waiting.
Open SQLite with _txlock=immediate and a 30s busy timeout, shrink the
pool, and retry locked upserts. Add a 32-way concurrent upsert test.
Ordinary
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