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; any other value shows
every event. Failed lists the events with a failed delivery, Pending
those with one pending or retrying, each once, under the full list's
50-row limit and order. A filtered list finds the matching deliveries
through `idx_deliveries_status`, looks up their events by ID and sorts
them, and reads the rows of only the events shown; its counts read the
deliveries alone. The line beside the heading says what a filter
counts, and Replay returns to the list it was pressed in. The README
and the comments naming the query parameters the service reads include
`show`.

Model: opus-5-5

internal/server/alpine_browser_test.go

@@ -943,15 +943,15 @@ 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
@@ -959,12 +959,12 @@ 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")

internal/server/sentry.go

@@ -219,8 +219,8 @@ func keptSentryHeaders(headers map[string]string) map[string]string {
 // sentryKeepsHeader reports whether a request header is routing or
 // content metadata rather than client-chosen payload. Referer is kept
 // on the reasoning that it is browser-set, that the only query
-// parameters in this service's own URLs are the login page's `next`
-// and `notice`, and that Referrer-Policy is set to
+// parameters in this service's own URLs are the login page's `next`,
+// `notice` and the event log's `show`, and that Referrer-Policy is set to
 // strict-origin-when-cross-origin. X-Request-Id ties the event to the
 // local access log line, which holds the rest of the detail.
 func sentryKeepsHeader(name string) bool {