Fix cryptic combine crash and segment ordering past 1000 files

[s0len]

While downloading a ~3300-segment Nextory audiobook with --combine, the combine step crashed with a confusing FileNotFoundError: output_file.aac and discarded the in-progress merge. Two distinct issues:

1. combine_audiofiles crashes cryptically when ffmpeg writes no output

The re-encode fallback in output.py could finish without producing output_file (e.g. a transient ffmpeg error on a chunk). The code then ran os.remove(tmp_input) followed by shutil.move(tmp_output, tmp_input) on a file that was never created — crashing with a FileNotFoundError and deleting the accumulated combine progress.

Now the fallback result is checked: if ffmpeg produced no output, raise FailedCombining (logging ffmpeg's stderr at debug level) and leave tmp_input intact, instead of crashing and losing progress.

2. Segment filenames under-pad past powers of ten

create_filepath used zfill(int(log10(len(files)))). For 3307 files that pads to 3 digits, so Part 1000 … Part 3306 sort lexically before Part 999, scrambling the concat order. + 1 makes the padding width cover the largest index.

Testing

Built and installed locally; a full --combine -f mp4 download of a 3307-segment book now completes end-to-end and produces a valid 2h19m mp4.

🤖 Generated with Claude Code

[s0len]

Looks good to me and verified locally. Thanks for your work @jo1gi