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/hub.go

@@ -78,21 +78,46 @@ func (h *Hub) logger() *slog.Logger {
 // whose queue is full is closed as a slow consumer. A payload that can't be
 // marshalled is logged and nothing is sent.
 func (h *Hub) Push(msgType string, payload any) {
-	var rawPayload json.RawMessage
+	h.push(
+		Envelope{
+			Type: msgType,
+		},
+		payload,
+	)
+}
+
+// PushEvent sends a TypeEvent message to every client, as Push does, with
+// eventType as the envelope's EventType.
+func (h *Hub) PushEvent(eventType string, payload any) {
+	h.push(
+		Envelope{
+			Type:      TypeEvent,
+			EventType: eventType,
+		},
+		payload,
+	)
+}
+
+// push sends envelope, with payload marshalled into it, to every client (see
+// Push).
+func (h *Hub) push(envelope Envelope, payload any) {
 	if payload != nil {
 		var err error
-		if rawPayload, err = json.Marshal(payload); err != nil {
-			h.logger().Error("marshal push payload", "type", msgType, "error", err)
+		envelope.Payload, err = json.Marshal(payload)
+		if err != nil {
+			h.logger().Error(
+				"marshal push payload",
+				"type", envelope.Type,
+				"event_type", envelope.EventType,
+				"error", err,
+			)
 			return
 		}
 	}
 
-	msg, err := json.Marshal(Envelope{
-		Type:    msgType,
-		Payload: rawPayload,
-	})
+	msg, err := json.Marshal(envelope)
 	if err != nil {
-		h.logger().Error("marshal push", "type", msgType, "error", err)
+		h.logger().Error("marshal push", "type", envelope.Type, "error", err)
 		return
 	}