refactor: self-contained delivery tasks — engine delivers without DB reads in happy path

The webhook handler now builds DeliveryTask structs carrying all target
config and event data inline (for bodies ≤16KB) and sends them through
the delivery channel. In the happy path, the engine delivers without
reading from any database — it only writes to record delivery results.

For large bodies (≥16KB), Body is nil and the engine fetches it from the
per-webhook database on demand. Retry timers also carry the full
DeliveryTask, so retries avoid unnecessary DB reads.

The database is used for crash recovery only: on startup the engine scans
for interrupted pending/retrying deliveries and re-queues them.

Implements owner feedback from issue #15:
> the message in the <=16KB case should have everything it needs to do
> its delivery. it shouldn't touch the db until it has a success or
> failure to record.

internal/handlers/handlers_test.go

@@ -22,7 +22,7 @@ import (
 // noopNotifier is a no-op delivery.Notifier for tests.
 type noopNotifier struct{}
 
-func (n *noopNotifier) Notify(delivery.Notification) {}
+func (n *noopNotifier) Notify([]delivery.DeliveryTask) {}
 
 func TestHandleIndex(t *testing.T) {
 	var h *Handlers