fix(irc): search by surname, and rank the answer by author (#1331)

[Kukkerem]

Fixes #1331.

A search bot ANDs every term against a filename, so the given name is the term
that empties the result set. Measured against irchighway's #ebooks: "Revelations
David Petrie" is answered "no results", "Revelations Petrie" returns 9 matches,
6 of which parse, all filed as "D Petrie".

The query now carries the title and the surname, read off the search variant so
the ISBN fallback and a manual query - which set author="" on purpose - keep
their current shape.

Title-only, the shape #1295 settled on for Prowlarr, does not transfer: the bot
caps an answer at 1000 matches, and a bare "Revelations" hits that cap with 923
parsed rows across 500 authors, so the cap itself can drop the wanted book. The
surname is the token the two spellings share and it keeps the answer small.

The full author then orders what comes back, reusing author_affinity from #1295,
since a surname also matches a different author who shares it. It sits under
server availability the way indexer priority does in #1295: a download addresses
one named bot and waits 120s for it, so a match from a bot that has left the
channel must not outrank a mismatch that can answer. Ranking runs on the way out
rather than before the cache, because one query identity is shared by every book
that produced that query.

Two things found while testing:

• The parser writes the literal "Unknown" when a filename has no " - " separator
  (parser.py:168). Ranked literally that sorts as a wrong author, so
  author_affinity's middle tier was unreachable here; it is now read as absent.
  5 of those 923 rows are affected.
• author_affinity moves to shelfmark/core/author_match.py, unchanged, so IRC
  does not import from the Prowlarr package. Prowlarr behaviour is untouched and
  its tests pass as they are.

The three IRC assertions in the #1252 regression file move to the surname form.
The invariant they pin - one contributor's name reaches the query, never the
whole credit list - is unchanged.

Tested with make python-lint, python-format, python-dead-code, python-typecheck
and python-test, and end to end against irchighway with the patched source: it
posts "Revelations Petrie" and returns 6 releases.