v2.4.1 — Multi-copy logout fixed, and probe failures now explain themselves

Multi-copy logout fixed, and probe failures now explain themselves. Two fixes from real-world reports on 2.4.0.

Fixed

• No more forced logout when running multiple subarr copies (#411). Two copies on the same host (say a TV stack and an Anime stack on one box, on different ports) shared a single session cookie, because browsers scope cookies by host and ignore the port, so signing into the second copy logged you out of the first. The session cookie name is now unique per instance, so both stay logged in. You will be asked to log in once after upgrading, since the cookie name changes.
• Eager-probe failures are now diagnosable (#416). When probing errored on every file, the log only showed a count and hid the actual reason (a path or permission mismatch caught before ffprobe ever ran). The run summary now reports the failure reason plus a sample, and escalates to a warning when the whole batch fails, which is the signature of a media-root or arr path-prefix misconfiguration.