Show 50 recent events with size, processing time and status (closes #347)

The recent events list on a webhook's page now shows the 50 newest
events, each with its time relative to now (the full UTC timestamp on
hover), its body size, its processing time and, when the webhook has
exactly one HTTP target, that target's last HTTP status, colour-coded.
Resubmitted copies are marked, as in the event log.

Each event now records its body's size in bytes when it is stored, on
receipt and on resubmit, and the list reads that instead of the body.
Processing time is read from the existing delivery and attempt
timestamps. The list loads its deliveries and their attempts in batched
queries of its own, reading only the columns it shows. A failed load
answers with an error instead of an empty list.

Model: opus-5-5

internal/database/model_event.go

@@ -31,6 +31,11 @@ type Event struct {
 	Body        string `gorm:"type:text"   json:"body"`
 	ContentType string `json:"contentType"`
 
+	// BodyBytes is the size of Body in bytes, recorded when the event
+	// is stored so the recent events list can show it without reading
+	// the body.
+	BodyBytes int64 `gorm:"not null" json:"bodyBytes"`
+
 	// ResubmittedFromID names the event this one was copied from by
 	// an operator resubmit. It is nil for an event that arrived on
 	// the receiver, which is every event created before the column