Add in-process event bus with ring buffer for workflow event broadcasting

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/claude-agent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/eventbus/bus.go

@@ -0,0 +1,116 @@
+package eventbus
+
+import "sync"
+
+type Event struct {
+	Seq           int64
+	WorkflowRunID string
+	AlbumID       string
+	Quality       string
+	EventType     string
+	Step          string
+	Message       string
+	Data          interface{}
+}
+
+type Subscription struct {
+	Ring *RingBuffer[*Event]
+	C    chan struct{}
+	done chan struct{}
+	once sync.Once
+}
+
+type EventBus struct {
+	mu     sync.RWMutex
+	topics map[string]map[*Subscription]struct{}
+	global map[*Subscription]struct{}
+}
+
+func New() *EventBus {
+	return &EventBus{
+		topics: make(map[string]map[*Subscription]struct{}),
+		global: make(map[*Subscription]struct{}),
+	}
+}
+
+func (b *EventBus) Publish(topic string, event *Event) {
+	b.mu.RLock()
+	defer b.mu.RUnlock()
+
+	if subs, ok := b.topics[topic]; ok {
+		for sub := range subs {
+			sub.Ring.Push(event)
+			select {
+			case sub.C <- struct{}{}:
+			default:
+			}
+		}
+	}
+
+	for sub := range b.global {
+		sub.Ring.Push(event)
+		select {
+		case sub.C <- struct{}{}:
+		default:
+		}
+	}
+}
+
+func (b *EventBus) Subscribe(topic string) (*Subscription, func()) {
+	sub := &Subscription{
+		Ring: NewRingBuffer[*Event](256),
+		C:    make(chan struct{}, 1),
+		done: make(chan struct{}),
+	}
+
+	b.mu.Lock()
+	if b.topics[topic] == nil {
+		b.topics[topic] = make(map[*Subscription]struct{})
+	}
+	b.topics[topic][sub] = struct{}{}
+	b.mu.Unlock()
+
+	cleanup := func() {
+		sub.once.Do(func() {
+			b.mu.Lock()
+			delete(b.topics[topic], sub)
+			if len(b.topics[topic]) == 0 {
+				delete(b.topics, topic)
+			}
+			b.mu.Unlock()
+			close(sub.done)
+		})
+	}
+
+	return sub, cleanup
+}
+
+func (b *EventBus) SubscribeGlobal() (*Subscription, func()) {
+	sub := &Subscription{
+		Ring: NewRingBuffer[*Event](256),
+		C:    make(chan struct{}, 1),
+		done: make(chan struct{}),
+	}
+
+	b.mu.Lock()
+	b.global[sub] = struct{}{}
+	b.mu.Unlock()
+
+	cleanup := func() {
+		sub.once.Do(func() {
+			b.mu.Lock()
+			delete(b.global, sub)
+			b.mu.Unlock()
+			close(sub.done)
+		})
+	}
+
+	return sub, cleanup
+}
+
+func (b *EventBus) HasTopic(topic string) bool {
+	b.mu.RLock()
+	defer b.mu.RUnlock()
+	_, ok := b.topics[topic]
+	return ok
+}