Function upsert_embedding 

async fn upsert_embedding(
    pool: &SqlitePool,
    chunk_id: &str,
    document_id: &str,
    model: &str,
    dims: usize,
    text_hash: &str,
    blob: &[u8],
) -> Result<()>

Upsert an embedding into both embeddings (metadata) and chunk_vectors (blob).

Uses INSERT ... ON CONFLICT DO UPDATE for idempotent writes.