Add rebuild tool: scan nginx volumes to reconstruct SQLite index

Walks nginx autoindex JSON recursively, merges replicas across volumes,
and bulk-inserts into a fresh database. Also adds reqwest json feature.

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

src/main.rs

@@ -45,8 +45,7 @@ async fn main() {
             axum::serve(listener, app).await.unwrap();
         }
         Commands::Rebuild => {
-            eprintln!("rebuild not yet implemented");
-            std::process::exit(1);
+            mkv::rebuild::run(&config).await;
         }
         Commands::Rebalance { dry_run: _ } => {
             eprintln!("rebalance not yet implemented");