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/watcher.go

@@ -0,0 +1,80 @@
+package main
+
+import (
+	"context"
+	"log"
+	"strings"
+
+	"fn-registry/functions/infra"
+)
+
+func (s *Server) watchLoop(ctx context.Context, root, kind string) {
+	ch, err := infra.WatchDirFsnotify(ctx, root)
+	if err != nil {
+		log.Printf("watcher %s: %v", root, err)
+		return
+	}
+	for ev := range ch {
+		if !strings.HasSuffix(ev.Path, ".md") {
+			continue
+		}
+		s.handleFsEvent(kind, ev)
+	}
+}
+
+func (s *Server) handleFsEvent(kind string, ev infra.FsEvent) {
+	switch kind {
+	case "issue":
+		s.handleIssueEvent(ev)
+	case "flow":
+		s.handleFlowEvent(ev)
+	}
+}
+
+func (s *Server) handleIssueEvent(ev infra.FsEvent) {
+	id := issueIDFromPath(ev.Path, s.issuesDir)
+	if id == "" {
+		return
+	}
+	if ev.Op == "remove" || ev.Op == "rename" {
+		s.deleteIssue(id)
+		s.hub.broadcast(SSEEvent{Type: "removed", ID: id, Path: ev.Path})
+		return
+	}
+	iss, _, err := infra.ParseIssueMd(ev.Path)
+	if err != nil {
+		log.Printf("parse %s: %v", ev.Path, err)
+		return
+	}
+	if err := s.upsertIssueRow(iss); err != nil {
+		log.Printf("upsert %s: %v", iss.ID, err)
+		return
+	}
+	s.hub.broadcast(SSEEvent{Type: "updated", ID: iss.ID, Path: ev.Path})
+}
+
+func (s *Server) handleFlowEvent(ev infra.FsEvent) {
+	id := issueIDFromPath(ev.Path, s.flowsDir)
+	if id == "" {
+		return
+	}
+	if ev.Op == "remove" || ev.Op == "rename" {
+		s.deleteFlow(id)
+		s.hub.broadcast(SSEEvent{Type: "flow_removed", ID: id, Path: ev.Path})
+		return
+	}
+	flows, err := infra.ScanFlowsDir(s.flowsDir)
+	if err != nil {
+		log.Printf("scan flows: %v", err)
+		return
+	}
+	for _, fl := range flows {
+		if fl.ID == id {
+			if err := s.upsertFlowRow(fl); err != nil {
+				log.Printf("upsert flow %s: %v", id, err)
+			}
+			s.hub.broadcast(SSEEvent{Type: "flow_updated", ID: id, Path: ev.Path})
+			return
+		}
+	}
+}