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).

.gitignore

@@ -0,0 +1,5 @@
+backend/kanban_cpp_backend
+backend/operations.db*
+backend/registry.db
+build/
+*.log