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

internal/handlers/handlers.go

@@ -64,7 +64,7 @@ type HandlersParams struct {
 	Healthcheck     *healthcheck.Healthcheck
 	Session         *session.Session
 	Middleware      *middleware.Middleware
-	Notifier        delivery.Notifier
+	Engine          *delivery.Engine
 	Archives        archive.Archives
 	CircuitBreakers delivery.CircuitBreakers
 	SSRFGuard       *delivery.Guard
@@ -82,7 +82,7 @@ type Handlers struct {
 	dbMgr     *database.WebhookDBManager
 	session   *session.Session
 	mw        *middleware.Middleware
-	notifier  delivery.Notifier
+	engine    *delivery.Engine
 	archives  archive.Archives
 	breakers  delivery.CircuitBreakers
 	mtr       *metrics.Set
@@ -138,7 +138,7 @@ func New(
 	s.dbMgr = params.WebhookDBMgr
 	s.session = params.Session
 	s.mw = params.Middleware
-	s.notifier = params.Notifier
+	s.engine = params.Engine
 	s.archives = params.Archives
 	s.breakers = params.CircuitBreakers
 	s.mtr = params.Metrics