GET /prompt_manager/settings adds settings.monitoring_enabled, a sources
block (configured roots with valid/code/reason, effective roots with their
listing index and watch state, auto_detected) and the monitor state
(present, observer_alive, scheduled). Every path is a public path.
A source is "watching" only while a live observer scheduled it, "manual"
when monitoring is off or the process has no monitor, and otherwise
"restart_pending".
POST accepts monitoring_enabled as a strict bool, saved under
gallery.monitoring.enabled; restart_required_fields names it when the
running observer does not already match. A save that changes the sources
runs as a "Source change" records hold, so it is refused with 409 busy
while a scan runs. Case variants of one folder are saved once.
POST /prompt_manager/settings/validate-source {path} checks a path
lexically (no "..", no control characters, native path forms only) and
then against the gallery-root rules, answering a code and reason, and
writes nothing.