feat(backend): scaffold state-based sync foundation (no GUI)

Add core scanner, diff engine, SQLite DB, queue, events, scheduler, utils
Wire settings + bootstrap to compute actions;

.vscode/settings.json

@@ -1,4 +1,5 @@
 {
+    "python.terminal.activateEnvironment": true,
     "python.testing.pytestArgs": [
         "tests"
     ],