Function find_pending_chunks 

async fn find_pending_chunks(
    pool: &SqlitePool,
    model: &str,
    limit: Option<usize>,
) -> Result<Vec<PendingChunk>>

Find chunks that are missing embeddings or have stale hashes.

A chunk is “pending” if:

1. No row exists in embeddings for this chunk+model, or
2. The stored hash differs from the chunk’s current hash.