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.

Processing time is how long the event's slowest delivery took, from
being queued to its last recorded attempt. It is read from the existing
delivery and attempt timestamps, so nothing new is recorded. The body's
size is read in SQL, never the body.

Deliveries and attempts load in one batched query each, reusing the
event log's attempt loader.

Model: opus-5-5

internal/handlers/delivery_result_view.go

@@ -1,6 +1,8 @@
 package handlers
 
 import (
+	"time"
+
 	"sneak.berlin/go/webhooker/internal/delivery"
 )
 
@@ -25,7 +27,7 @@ const maxRenderedResponseBytes = 4096
 // cut, so an oversized stored response never becomes a Go
 // string at all.
 const deliveryResultColumns = "delivery_id, attempt_num, success, " +
-	"status_code, error, duration, " +
+	"status_code, error, duration, created_at, " +
 	"substr(cast(response_body as blob), 1, ?) AS response_body, " +
 	"length(cast(response_body as blob)) AS response_bytes"
 
@@ -106,6 +108,10 @@ type deliveryResultRow struct {
 	Duration      int64
 	ResponseBody  []byte
 	ResponseBytes int64
+
+	// CreatedAt is when the attempt's result was recorded, which
+	// is when the attempt finished.
+	CreatedAt time.Time
 }
 
 // view projects a loaded row for rendering, stripping the