Background queue that batches payloads and flushes them every 100 ms.

Payloads for each URL key are accumulated in the GenServer state and sent in batches of up to 4 via DirectWriter on each flush tick. Batches (across all URL keys) are flushed concurrently, bounded by EndPointBlank.Config.worker_count/0 (default 4), so a flush tick never fires more than that many writes in flight at once.

Each URL key's queue is capped at 1000 payloads. If an intake outage (or any slow/hung downstream) causes payloads to accumulate faster than they can be flushed, the oldest payloads for that key are dropped to keep memory usage bounded in the host app.