Store events and create deliveries in the delivery engine (closes #544)

The receiver, resubmit and replay handlers now read and check the
request and make one call. Engine.StoreEvent stores an event with a
pending delivery for each active target and the totals in one
transaction, then queues them; Engine.CreateReplay does the same for
one replay delivery. The handlers take *delivery.Engine, so the
Notifier interface and its bridge are gone, and the engine's notify is
no longer exported. Archives and CircuitBreakers stay because the
handler tests fake them. The tests that read a queued task moved to
the engine's tests; the handler tests use an engine that is never
started.

Model: opus-5-5

cmd/webhooker/main.go

@@ -211,9 +211,6 @@ func newApp() *fx.App {
 			delivery.New,
 			archive.NewWriters,
 			archive.NewSweeper,
-			// Wire *delivery.Engine as delivery.Notifier so the
-			// webhook handler can notify the engine of new deliveries.
-			func(e *delivery.Engine) delivery.Notifier { return e },
 			// Wire *archive.Writers as archive.Archives so deleting
 			// or renaming a webhook or target reaches its archive
 			// files.