inspect_kindle_file.py: check a .mobi, .azw, or .azw3 file before translating it
==================================================================================

What it does
  Reads only the file headers and prints the title, the format generation
  (MOBI 6, KF8/AZW3, or a joint file with both), the text encoding, the
  compression, the encryption field, and the language tag. It never changes
  the file. It needs Python 3 and nothing else.

  The encryption field is the same field BookTranslator's MOBI reader checks:
  any value other than 0 means the file is DRM-protected and will be rejected.

Usage
  python3 inspect_kindle_file.py book.mobi
  python3 inspect_kindle_file.py *.azw3

Test files (checked 2026-10-29)
  Franz Kafka, "Die Verwandlung", Project Gutenberg eBook #22367 (public domain
  in the US). Download the files from Project Gutenberg rather than from us:
    Older Kindle file (MOBI 6): https://www.gutenberg.org/ebooks/22367.kindle.images
    Newer Kindle file (KF8):    https://www.gutenberg.org/ebooks/22367.kf8.images
    EPUB3:                      https://www.gutenberg.org/ebooks/22367.epub3.images

  SHA-256 of the files we downloaded:
    8d0a2b47b394c94f72668d1314ad1d4a76bb0c91b2cc924a07cb93e64ac7967a  22367.kindle.images
    e37df33abeb714439ca76489f4894b20e2b519e262598105e9ff322476109483  22367.kf8.images
    bfd98560075632c0b805720a00c3143044df38dae447e8c3c04f117bc4560d25  22367.epub3.images
  Project Gutenberg rebuilds its files from time to time, so a later download
  may have different hashes.

  The fourth test file is a copy of the MOBI 6 file with one change: the
  2-byte encryption field (offset 12 of record 0) set to 2. It simulates a
  DRM-flagged file. It is not a real store purchase and cannot be opened by a
  reader app. Make it yourself with:
    python3 - <<'PY'
    import struct
    d = bytearray(open("22367.kindle.images", "rb").read())
    off = struct.unpack(">I", d[78:82])[0]
    d[off + 12:off + 14] = struct.pack(">H", 2)
    open("drm-flag-set.mobi", "wb").write(d)
    PY

Output we got (files saved as die-verwandlung.mobi, die-verwandlung.azw3,
drm-flag-set.mobi, and die-verwandlung.epub)
  == die-verwandlung.mobi
  title: Die Verwandlung
  format: MOBI 6
  text encoding: UTF-8
  compression: PalmDOC
  encryption: none
  language: de
  verdict: OK to upload

  == die-verwandlung.azw3
  title: Die Verwandlung
  format: KF8 (AZW3)
  text encoding: UTF-8
  compression: PalmDOC
  encryption: none
  language: de
  verdict: OK to upload

  == drm-flag-set.mobi
  title: Die Verwandlung
  format: MOBI 6
  text encoding: UTF-8
  compression: PalmDOC
  encryption: Mobipocket DRM
  language: de
  verdict: DRM-protected: BookTranslator will reject it

  == die-verwandlung.epub
  ZIP container (EPUB or KFX-ZIP), not a MOBI-family file