fix(video): a Specials/Bonus folder's episodes were matched to TMDB as unrelated standalone movies

Found live: a "Specials" folder full of one-off-named bonus episodes had
every file appear as its own poster, matched against TMDB by its own
title, because guessit finds no season/episode grammar at all in a
filename with no SxxExx of its own — so the classification (episode vs
standalone movie), based solely on that, fell to the movie branch. Real,
unrelated films happened to share several of those one-off titles and
matched confidently, one per Special, cluttering the Videos view with
dozens of wrong posters instead of grouping under the show.

An ancestor folder saying this is part of a show — a numbered season, or
Specials/Bonus/Extras -> season 0 — is now trusted over the filename
having no SxxExx of its own. The show's name can't come from this file's
own guessit title (that's the bug) or from siblings in the same Specials
folder (every one of them has the same gap) — it's borrowed from the
show's ordinary season folders next door, which do carry it in the usual
ShowName.SxxExx shape (_title_from_show_siblings). A synthetic, stable
episode number (alphabetical rank among the folder's video files) stands
in for the real one nothing in a Specials folder provides.

Generic, not specific to a folder literally named "Specials": the same
fallback fires for any season-like ancestor folder whose files lack
per-file episode grammar, numbered seasons included (covered by a
dedicated test).

Also hardens _title_from_siblings to require the sibling's own episode
number too, not just a title — otherwise it would borrow one Special's
one-off title as if it were representative, on a folder like this one.

Existing already-indexed entries will need a rescan (restart the node) to
be re-enriched under this logic — nothing re-derives them on its own.

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