DoS / resource: no explicit HTTP body limit; background monitor scans many keys every interval

## Summary

Two related scalability concerns:

1. **JSON-RPC body size:** The server relies on library defaults for maximum request body size. A client could send very large JSON payloads to increase memory and CPU cost during parsing.

2. **Background loop:** Every 10 seconds the monitor loads up to 1000 requests (`get_requests(Some(1000))`) and iterates (`src/rpc.rs`). As the database grows, work per tick grows unless pagination or indexing by status is introduced.

## Suggested mitigations

- Configure a strict max body size at the HTTP layer or reverse proxy.
- Query only pending/processing keys (secondary index or prefix scan filtered by status) instead of scanning recent N requests.

## References

- `src/rpc.rs` (tokio spawn monitor)
- `src/storage.rs` (`get_requests`)