Event log: only the newest event expanded, and only the 50 most recent (closes #349)

The event log now loads the 50 newest events in its query and opens
only the newest on load, as the recent events on a webhook's page
already do. With 50 at most there is never a second page, so paging is
gone: the page links, the `page` query parameter, the page number that
Replay and Resubmit carried back to the log, and `pageOrFirst` with its
test. The count beside the heading reads "50 most recent of N events"
when there are more. The comments and README lines that named `page` as
the query parameter the service reads now name `next` and `notice`.

Model: opus-5-5

internal/handlers/source_logs_test.go

@@ -2,9 +2,12 @@ package handlers_test
 
 import (
 	"context"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
+	"time"
 
 	"github.com/go-chi/chi"
 	"github.com/stretchr/testify/assert"
@@ -153,3 +156,55 @@ func TestHandleSourceLogs_MasksSlackWebhookURL(t *testing.T) {
 	assert.Contains(t, body, tgt.Name)
 	assert.Contains(t, body, "delivered")
 }
+
+// TestHandleSourceLogs_ShowsFiftyNewestEvents proves the event log
+// holds the 50 newest events, newest first, and not one more, and says
+// how many events there are in all.
+func TestHandleSourceLogs_ShowsFiftyNewestEvents(t *testing.T) {
+	t.Parallel()
+
+	f := newRecentEventsFixture(t)
+	base := time.Now().Add(-time.Hour)
+
+	for i := range 51 {
+		f.event(
+			t, fmt.Sprintf("application/x-log-%02d", i), "{}",
+			base.Add(time.Duration(i)*time.Second),
+		)
+	}
+
+	body := renderSourceLogsPage(t, f.h, f.sess, f.webhook.ID)
+
+	assert.Equal(t, 50, strings.Count(body, `role="button"`))
+	assert.NotContains(t, body, "application/x-log-00")
+	assert.Contains(t, body, "application/x-log-01")
+	assert.Less(
+		t,
+		strings.Index(body, "application/x-log-50"),
+		strings.Index(body, "application/x-log-49"),
+	)
+	assert.Contains(t, body, "50 most recent of 51 events")
+}
+
+// TestHandleSourceLogs_OnlyNewestStartsExpanded proves that of the
+// events in the log only the newest starts expanded.
+func TestHandleSourceLogs_OnlyNewestStartsExpanded(t *testing.T) {
+	t.Parallel()
+
+	f := newRecentEventsFixture(t)
+	now := time.Now()
+
+	f.event(t, "application/x-older", "{}", now.Add(-time.Minute))
+	f.event(t, "application/x-newer", "{}", now)
+
+	body := renderSourceLogsPage(t, f.h, f.sess, f.webhook.ID)
+
+	assert.Equal(t, 1, strings.Count(body, " data-open>"))
+
+	open := strings.Index(body, " data-open>")
+	newer := strings.Index(body, "application/x-newer")
+	older := strings.Index(body, "application/x-older")
+
+	assert.Less(t, open, newer, "the newest event is not the open one")
+	assert.Less(t, newer, older)
+}