feat: initial scaffold of kanban_cpp v2 (issue 0130)

Frontend C++ ImGui (main.cpp + 4 paneles) + backend Go (HTTP + SQLite + fsnotify + SSE).
Reusa parse/scan/watch funcs del registry (issue 0130a).

backend/sse_hub.go

@@ -0,0 +1,46 @@
+package main
+
+import (
+	"sync"
+)
+
+type SSEEvent struct {
+	Type string `json:"type"`
+	ID   string `json:"id"`
+	Path string `json:"path,omitempty"`
+}
+
+type SSEHub struct {
+	mu      sync.RWMutex
+	clients map[chan SSEEvent]struct{}
+}
+
+func newSSEHub() *SSEHub {
+	return &SSEHub{clients: map[chan SSEEvent]struct{}{}}
+}
+
+func (h *SSEHub) subscribe() chan SSEEvent {
+	ch := make(chan SSEEvent, 16)
+	h.mu.Lock()
+	h.clients[ch] = struct{}{}
+	h.mu.Unlock()
+	return ch
+}
+
+func (h *SSEHub) unsubscribe(ch chan SSEEvent) {
+	h.mu.Lock()
+	delete(h.clients, ch)
+	h.mu.Unlock()
+	close(ch)
+}
+
+func (h *SSEHub) broadcast(ev SSEEvent) {
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+	for ch := range h.clients {
+		select {
+		case ch <- ev:
+		default:
+		}
+	}
+}