- backfill quarantine_days<=0 to 3 with migration 0006
- reject quarantine_enabled with quarantine_days<1 on create and update
- cap upstream Last-Modified at now
- answer 503 with Retry-After when the first-seen lookup fails
- refuse uncached immutable paths published under quarantine_days ago with a JSON 403
- take publish time from Last-Modified, else a Redis first-seen key
- let goproxy virtual probes skip a quarantining member
- default quarantine_days to 3 on create and keep it on update
- document quarantine in caching.md