feat(fanart-tv): improve TV show support — Custom Scraper integration, season banners, TMDb search

[Arny80Hexa]

The Custom TV Scraper settings currently only offer TMDb Images for artwork fields (poster, fanart, banner, etc.). Fanart.tv provides high-quality artwork and is already integrated as an image provider in MediaElch, but is not available in the Custom TV Scraper dropdowns.

Proposal: Add Fanart.tv as an additional image source option in the Custom TV Scraper settings widget, alongside the existing TMDb Images. Each provider should only appear for fields it actually supports.

Analyzed with AI assistance (Claude Opus 4.6).

[Arny80Hexa]

Implementation note: Fanart.tv's TV API only accepts TvDb IDs — TMDb IDs and IMDB IDs are not supported for TV show lookups. Since TheTvDb is being removed (#1120), FanartTv will obtain the TvDbId transparently via a TMDb external_ids lookup whenever a TvDbId is not already available on the TvShow object. This ensures FanartTv works regardless of which scraper is configured (IMDB, TMDb, OMDb, or any combination in the Custom Scraper).

For movies, FanartTv accepts both TMDb IDs and IMDB IDs directly — no extra lookup needed.