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/messages.go

@@ -19,23 +19,27 @@
 //	{"type":"echo","id":"2","payload":{...}}
 //	{"type":"error","id":"3","error":"..."}
 //	{"type":"item.created","payload":{...}}
+//	{"type":"event","eventType":"OpportunitiesUpdated","payload":{...}}
 //
-// Any other type from a client gets an "unknown message type" error. The last
-// message is one sent by Hub.Push: its type is the pushed message type.
+// Any other type from a client gets an "unknown message type" error. The
+// last two are sent by Hub.Push, with the pushed message type, and by
+// Hub.PushEvent, with TypeEvent and the event's type (see package events).
 package websocket
 
 import "encoding/json"
 
 // Envelope is the wire format for every message in both directions.
+// EventType is set only on a TypeEvent message, naming the event.
 type Envelope struct {
-	Type    string          `json:"type"`
-	ID      string          `json:"id,omitempty"`
-	Payload json.RawMessage `json:"payload,omitempty"`
-	Error   string          `json:"error,omitempty"`
+	Type      string          `json:"type"`
+	EventType string          `json:"eventType,omitempty"`
+	ID        string          `json:"id,omitempty"`
+	Payload   json.RawMessage `json:"payload,omitempty"`
+	Error     string          `json:"error,omitempty"`
 }
 
-// Envelope types. TypeEvent, TypeRequest and TypeAck are reserved: nothing
-// sends or handles them yet.
+// Envelope types. TypeRequest and TypeAck are reserved: nothing sends or
+// handles them yet.
 const (
 	// TypeWelcome is sent to a client when it connects, with its client ID.
 	TypeWelcome = "welcome"
@@ -45,7 +49,8 @@ const (
 	TypePong = "pong"
 	// TypeEcho is sent back to the client with the same ID and payload.
 	TypeEcho = "echo"
-	// TypeEvent is reserved for events pushed to clients.
+	// TypeEvent is an event pushed to clients by Hub.PushEvent, named by the
+	// envelope's EventType.
 	TypeEvent = "event"
 	// TypeRequest is reserved for requests from clients.
 	TypeRequest = "request"