Fix three poster bugs in edititem: rematch skips poster update, TVDB cache key collision, movie poster lookup wrong path

[Copilot]

After the TMDB refactor, the edit-item screen has three independent poster bugs: rematching a show/movie never updates the poster image, manually selecting a different TVDB poster often silently returns the cached wrong one, and movie posters don't appear in the edit screen at all until a new poster is downloaded.

Changes

• Rematch now downloads and applies the poster (handleApplyTmdbRematch / handleApplyTvdbRematch)
  Both handlers updated the provider ID but explicitly skipped poster changes. Now downloads the poster from the matched result and calls applyPoster immediately. Failure is caught as a warning so it doesn't abort the rematch.

• TVDB poster cache key includes URL-derived suffix (downloadTvdbPoster)
  Cache file was keyed only on TVDB series ID, so any second poster selection for the same series returned the first cached file. Key is now {tvdbId}_{url_filename} so each distinct artwork URL gets its own file.

• Movie poster lookup uses parsedPath not path (currentLibraryPoster)
  m.path is the raw encoded SAF URI; itemKey contains the decoded parsedPath. They never matched, so the poster always appeared blank. Changed to m.parsedPath, consistent with every other movie lookup in the file.

// Before — encoded m.path never equals the decoded itemKey segment
movies.find((m) => m.path === path)?.poster

// After
movies.find((m) => m.parsedPath === path)?.poster

Fix #182