Constant UPSERT_CHECKPOINT_BLOBS_SQL 

pub const UPSERT_CHECKPOINT_BLOBS_SQL: &str = r#"
INSERT INTO checkpoint_blobs (thread_id, checkpoint_ns, channel, version, type, blob)
VALUES (?1, ?2, ?3, ?4, ?5, ?6)
ON CONFLICT (thread_id, checkpoint_ns, channel, version) DO NOTHING
"#;

Upsert checkpoint blobs. Blobs are immutable per (channel, version), so on conflict we keep the existing row.