Phase 1 security audit follow-up:
- Reject HLS session IDs that aren't safe filesystem components
(regex allowlist) to defend against path traversal via a buggy or
compromised server. Applied at StartHLSSession and at the /hls URL
handler; invalid IDs share the 404 of unknown sessions so the
accepted format isn't enumerable.
- /health no longer leaks the active filename, taskID prefix or client
IP to non-loopback callers. Uses net.IP.IsLoopback so IPv4-mapped
IPv6 (::ffff:127.0.0.1) is recognised and the empty-string parse
failure stops bypassing the boundary.
- unrar/7z passwords now travel through stdin instead of -p<password>
in argv, removing /proc/<pid>/cmdline disclosure. Control characters
in the password are rejected up front so a hostile NZB cannot feed
extra prompt answers. Both invocations are bounded by a 30-minute
context to stop indefinite hangs if the tool ever decides to prompt.
