v0.1.12

Flibusta covers: match extensionless entries + diagnostic logging

The real reason covers stayed blank: inside covers/<archive>.zip the
cover files are named just by book id with NO extension (e.g. 294098),
but we only matched <id>.jpg. Now we match an entry whose stem equals
the id and sniff the image type from its magic bytes (defaulting to
JPEG). Verified against an extensionless-entry archive.

Also much better diagnostics: library.Open now reports exactly what it
tried — which archive variants exist on disk and, when an archive opens
but the file isn't inside, the entry count and a few real names (so a
naming mismatch is obvious). The read/cover/download handlers log these
at WARN/INFO. Example: 'entry "999999.fb2" not found among 3 entries
(e.g. 836016.fb2, 836017.fb2); …zip: not on disk'.