A BACKEND_EXHAUSTED classification (stage A) now puts that profile's
credential into a BackendQuarantine for a configurable cooldown. A spawn
onto a quarantined profile is refused naming the credential and roughly
when it lifts; weighted/round-robin/fixed placement skip a quarantined
candidate; the quarantine lifts itself on the injected clock; and it is
visible on bridge_profiles.
Keyed by credential, not by profile name, via the new Profile.credentialId
(profiles sharing one credential quarantine together — e.g. two models on
one account) and effectiveCredentialId() (unset ⇒ quarantines alone,
today's behaviour unchanged). Fixed a related gap along the way: a reload
changing exhaustedPattern was silently reported "applied" even though it's
deferred — sameLaunchSettings() now catches it too.
Dai Ha