Added an events bus, and pushed OpportunitiesUpdated to WebSocket clients when the fetcher stores new opportunities

internal/events decouples the parts of JobSearch that cause events from
the parts that react to them. bus.go has Bus: Subscribe, and a synchronous
Publish that calls each handler in turn on the publisher's goroutine, so
handlers mustn't block (the hub doesn't: it queues per client). events.go
has the event types and their payloads, starting with OpportunitiesUpdated.

With fetcher.WithEvents, a save that stores new opportunities, or new
releases of stored ones, publishes OpportunitiesUpdated with their UUIDs and
counts. An opportunity whose row is only rewritten, as when it is fetched
again from the newest stored release, isn't counted as updated, and nothing
is published when nothing changed. store.SaveResult now reports
NewOpportunity for each opportunity saved.

job-search serve subscribes its WebSocket hub to the bus, so every client
gets {"type":"event","eventType":"OpportunitiesUpdated","payload":{"new":
[...],"updated":[...],"summary":{"new":n,"updated":n}}}. The Envelope gains
an eventType field, and Hub.PushEvent and Server.PushEvent send events.

internal/websocket/http_test.go

@@ -143,6 +143,38 @@ func TestPushReachesBothTransports(t *testing.T) {
 	}
 }
 
+// PushEvent sends a TypeEvent message naming the event, to both transports.
+func TestPushEvent(t *testing.T) {
+	h, addr := tcpHub(t, nil)
+	url := wsHub(t, h)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+	conn := dialWS(t, ctx, url)
+	c := dialTCP(t, addr)
+	c.read() // welcome
+	waitCount(t, h, 2)
+
+	h.PushEvent("OpportunitiesUpdated", map[string]int{"new": 1})
+	var e Envelope
+	if err := wsjson.Read(ctx, conn, &e); err != nil {
+		t.Fatal(err)
+	}
+	received := map[string]Envelope{
+		"websocket": e,
+		"tcp":       c.read(),
+	}
+	for transport, got := range received {
+		if got.Type != TypeEvent || got.EventType != "OpportunitiesUpdated" ||
+			string(got.Payload) != `{"new":1}` {
+			t.Errorf(
+				"%s: got %+v; want the OpportunitiesUpdated event",
+				transport, got,
+			)
+		}
+	}
+}
+
 // A payload that can't be marshalled is not sent.
 func TestPushBadPayload(t *testing.T) {
 	h, addr := tcpHub(t, nil)