Reapply "Build post chat and attachment workflows"

This reverts commit 5bc4a551af.

backend/internal/calendar/chat_events.go

@@ -0,0 +1,55 @@
+package calendar
+
+import "sync"
+
+type ChatEventHub struct {
+	mu          sync.Mutex
+	subscribers map[string]map[chan struct{}]struct{}
+}
+
+func NewChatEventHub() *ChatEventHub {
+	return &ChatEventHub{
+		subscribers: map[string]map[chan struct{}]struct{}{},
+	}
+}
+
+func (h *ChatEventHub) Subscribe(itemID string, channel string) (chan struct{}, func()) {
+	key := chatEventKey(itemID, channel)
+	events := make(chan struct{}, 1)
+
+	h.mu.Lock()
+	if h.subscribers[key] == nil {
+		h.subscribers[key] = map[chan struct{}]struct{}{}
+	}
+	h.subscribers[key][events] = struct{}{}
+	h.mu.Unlock()
+
+	return events, func() {
+		h.mu.Lock()
+		defer h.mu.Unlock()
+
+		delete(h.subscribers[key], events)
+		if len(h.subscribers[key]) == 0 {
+			delete(h.subscribers, key)
+		}
+		close(events)
+	}
+}
+
+func (h *ChatEventHub) Publish(itemID string, channel string) {
+	key := chatEventKey(itemID, channel)
+
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	for events := range h.subscribers[key] {
+		select {
+		case events <- struct{}{}:
+		default:
+		}
+	}
+}
+
+func chatEventKey(itemID string, channel string) string {
+	return itemID + ":" + channel
+}