The chat Hub's Shutdown() only closed the done channel and returned
immediately, racing against goleak.VerifyNone in TestHub_*. Worse, the
broadcast saturation path spawned a fire-and-forget goroutine to send
on the unregister channel, which could leak if Run() exited mid-flight.
Fix:
- Add `stopped` channel closed by Run() on exit; Shutdown() waits on it.
- Buffer `unregister` (256) and replace the anonymous goroutine with a
non-blocking select. Worst case the client is reaped on its next
failed broadcast attempt.
- handler_messages_test.go's setupTestHandler started a Hub but never
shut it down, leaking Run() goroutines into the hub_test.go run that
followed. Register t.Cleanup(hub.Shutdown) and close the gorm sqlite
connection too — the connectionOpener goroutine was the secondary leak.
