Periodically runs Outbox.Dispatcher.run/0 in a supervised Task.

Oban's free Cron plugin is minute-granularity, but the bus needs sub-minute dispatch latency. This GenServer ticks every interval_ms (default 5000) and spawns a Task on Outbox.TaskSupervisor to do the dispatch work.

Why a Task (not an Oban job):

• No per-tick Oban telemetry log noise.
• The dispatcher is internal infrastructure — Oban's retry/DLQ machinery doesn't add value here. The next tick (5s later) is the retry.
• Task crashes are isolated from the Ticker (supervised).

Concurrent overlap is safe: FOR UPDATE SKIP LOCKED in the dispatcher ensures two simultaneous runs never claim the same event.

Disabled in test (set enabled: false); tests drive dispatch explicitly via Outbox.Testing.with_sync_dispatch/1.