feat: verify acceptance criteria and document sync config

Add a scan->notify->web end-to-end smoke test against a seeded :memory: DB,
fix .gitignore so the cmd/naviwatcher source dir is not excluded by the
root binary rule, and document the new sync.interval field in config.yaml.example.

.gitignore

@@ -1,5 +1,5 @@
 .DS_Store
-naviwatcher
+/naviwatcher
 naviwatcher-linux
 naviwatcher-mac
 naviwatcher.exe