Mailpit: Concurrent map read & write in proxy CSS rewriter - remote unauth crash (fatal error: concurrent map read and map write)

Summary The screenshot/print proxy /proxy?data=… maintains a package-level assets mapstringMessageAssets cache, but reads the map without holding assetsMutex while a long-running cleanup goroutine and re-entrant CSS-rewriting code path concurrently write to it under the lock. When the...