feat(events): trace event publishing at verbose level

[tgiachi]

Summary

The event bus was silent on the happy path (it only logged listener failures and slow listeners). Now every publish emits a Verbose line with the event type and listener count:

[17:52:03 VRB] Publishing EngineStartedEvent to 2 listener(s)

• Verbose level (= logger.MinimumLevel: Trace), consistent with how SQL statements are traced by the database package - invisible by default, zero noise on hot buses.
• Guarded by IsEnabled(LogEventLevel.Verbose) so the hot path pays nothing when disabled.
• The 0 listener(s) case is logged too - handy for spotting missing subscriptions.

Test Plan

• New test: at Verbose, publishes trace both a subscribed event (1 listener) and an unsubscribed one (0 listeners)
• Full suite green (single SQS load-flake, passed isolated and on rerun); build 0 errors