328 more routes used a JSON body as a dict straight from
request.get_json(), so a body of null, [] or a bare string crashed them
into a 500. Bare reads become json_object(); the
'get_json(silent=True) or {}' reads of action routes become
json_object(optional=True), which still treats a missing body as {}.
Routes that take arrays or pass the body through untouched are left
alone. The full backend suite shows no new failures.
Also carries three files' bare section citations (apps, backups,
notifications) and the extended citation guard, which needed them.
Extensions can now contribute walkthroughs via contributions.walkthroughs
in plugin.json. The backend validates definitions against a shared
contract (route, signal, manual, and check completion types), transports
them through the dynamic contribution envelope, and persists custom
walkthroughs from the Studio. Includes the JSON Schema, SDK version gate,
and extension scaffold support.