Event log: show attempt and delivery times, label replays, zone event times (closes #386)

Each attempt shows when it was recorded and each delivery when it was
created, in the event log and on the event's page. The event log's
event times read as the recent events list does: how long ago, with
the full UTC time on hover.

A delivery records whether Replay created it, in a new replay column
added to the delivery model in place. Such a delivery is labelled a
replay in the event's summary line and in both pages' delivery lists.
Both pages draw a delivery's row from one shared template.

Model: opus-5-5

internal/database/model_delivery.go

@@ -56,6 +56,10 @@ type Delivery struct {
 	// the index.
 	FinishedAt *time.Time `gorm:"index:idx_deliveries_status,priority:3" json:"finishedAt,omitempty"`
 
+	// Replay is set on a delivery created by the event log's Replay
+	// action, so the pages can tell it from the delivery it repeats.
+	Replay bool `gorm:"not null;default:false" json:"replay"`
+
 	// Relations. No model marshals the record it belongs to:
 	// Event.Deliveries and Target.Deliveries lead back here, and the
 	// JSON could loop.