Fix cluster DB pool size to 2 (sequential access, not concurrency-bound)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/rbv-cli/src/commands/cluster.rs

@@ -4,7 +4,7 @@ use rbv_cluster::{cluster_faces, ClusterConfig};
 use crate::args::ClusterArgs;
 
 pub async fn run(args: ClusterArgs) -> Result<()> {
-    let pool = rbv_data::connect(&args.database, args.concurrency as u32 + 2).await?;
+    let pool = rbv_data::connect(&args.database, 2).await?;
 
     info!("Loading unassigned face embeddings...");
     let faces = rbv_data::face::unassigned_face_embeddings(&pool).await?;