v2.4.0

Some Fetch boxes list a finished recording with size -1, so fetchtv treated it as still recording and never saved it.

• For each recording listed with no size, fetchtv now asks the box for the file length twice, 3 seconds apart. A steady length means the recording is finished, and it saves as normal.
• New growing: true when fetchtv is sure a recording is in progress: the box gives its live-recording size, or the length grew between the two checks.
• New unavailable: true when the box lists a recording but answers 404 or 410 for its file (for example, a recording deleted on the box).
• fetchtv save gives such a recording the new status unavailable with a one-line message. It does not count as a failure, so the exit code stays 0.
• --json output now always includes growing and unavailable.
• The README has a note on VPNs: a VPN that sends all traffic through its tunnel can block discovery; allow local network access or pass --ip.

What changes for existing users: recordings that used to stay "still recording" forever may now save. A deleted-on-box recording reports unavailable instead of still_recording.