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

The event log gains three links above the list: All, Failed (N) and Pending (N), carried in a `show` query parameter (`failed`, `pending`); any other value shows every event. Failed lists the events with at least one failed delivery, Pending those with one pending or retrying, under the same 50-row limit and newest-first order as the full list. Events are picked by their deliveries' status through `idx_deliveries_status`; each count is the number of distinct matching events, read from the deliveries alone, and can exceed the 50 shown. An empty filtered list says that no event matches. The README and the comments that name the query parameters the service reads now include `show`.

Model: opus-5-5

internal/server/alpine_browser_test.go

@@ -929,15 +929,15 @@ 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
@@ -945,12 +945,12 @@ 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")