Carry the event's receipt time into every delivery (closes #257)

Every Slack and Mattermost message the engine sent rendered a
`*Timestamp:*` of `0001-01-01T00:00:00Z` while the stored event's
`created_at` was correct. Both delivery paths reconstruct the event
from the Task that carries it, and no Task carries a receipt time, so
`FormatSlackMessage` formatted a zero `time.Time`.

`Task` is not the place to fix it: it is built in three places, two of
them on the receiver side, and a field there would have left the live
first-attempt and retry paths still zero while only the restart
recovery path came out correct. The stored row stays the single source
of truth instead. `resolveEventBody` becomes `hydrateEvent` and reads
`created_at` alongside the body, so every path that reconstructs an
event gets the receipt time with it.

A task that inlined its body previously never read the event row. It
does now, and a read failure there is no longer fatal: the row can be
reaped by retention while a queued delivery still holds its body, and
such a delivery goes out with the timestamp unset rather than being
dropped. A task with no inlined body still fails, as before.

internal/delivery/export_test.go

@@ -151,6 +151,16 @@ func (e *Engine) ExportProcessRetryTask(
 	e.processRetryTask(ctx, task)
 }
 
+// ExportEventForTask exposes the event reconstruction the delivery
+// paths run: buildEventFromTask followed by hydrateEvent.
+func (e *Engine) ExportEventForTask(
+	webhookDB *gorm.DB, task *Task,
+) (database.Event, error) {
+	return e.hydrateEvent(
+		webhookDB, buildEventFromTask(task), task,
+	)
+}
+
 // ExportProcessDelivery exposes processDelivery.
 func (e *Engine) ExportProcessDelivery(
 	ctx context.Context,