Event log: show only the events with a failed or a pending delivery (closes #390)

The event log had no way to list only the events whose delivery failed, and once it showed only the 50 newest, an older failure could not be found at all. It now has All, Failed (N) and Pending (N) links, carried in a `show` query parameter, so they work without the page's script library. Each filtered list keeps the 50-row limit and newest-first order, and lists an event once. It finds matching deliveries through `idx_deliveries_status` and looks their events up by ID, so its cost follows the matches, not the webhook's size. Replay returns to the list it was pressed in. The heading line says what a filter counts.

Model: opus-5-5

internal/server/alpine_browser_test.go

@@ -947,15 +947,15 @@ func checkEventSelection(
 }
 
 // checkEventKeyboard loads the event log and checks that Tab from the
-// page's Back link reaches the row of the newest event, the first after
-// it, and that Enter then collapses that event, which starts expanded,
-// and Space expands it again.
+// page's Pending link, the last link above the list, reaches the row of
+// the newest event, the first after it, and that Enter then collapses
+// that event, which starts expanded, and Space expands it again.
 func checkEventKeyboard(
 	ctx context.Context, t *testing.T, url, eventID string,
 ) {
 	t.Helper()
 
-	back := `//a[contains(text(), "Back to")]`
+	pending := `//a[starts-with(text(), "Pending")]`
 	expanded := `form[action$="/` + eventID + `/resubmit"]`
 
 	var focused string
@@ -963,12 +963,12 @@ func checkEventKeyboard(
 	require.NoError(t, chromedp.Run(
 		ctx,
 		loadPage(url),
-		chromedp.Focus(back, chromedp.BySearch),
+		chromedp.Focus(pending, chromedp.BySearch),
 		chromedp.KeyEvent(kb.Tab),
 		chromedp.Evaluate(`document.activeElement.textContent`, &focused),
 	))
 	require.Contains(t, focused, eventID,
-		"Tab from the Back link does not reach the event's row")
+		"Tab from the Pending link does not reach the event's row")
 
 	require.NoError(t, chromedp.Run(ctx, chromedp.KeyEvent(kb.Enter)))
 	assert.True(t, hidden(ctx, expanded), "Enter does not collapse the event")