feat(fanart-tv): improve TV show support and add fallback mechanism

[Arny80Hexa]

Summary

• TV show search: TheTvDb → TMDb — FanartTv's TV show search now uses TMDb instead of the deprecated TheTvDb scraper, with automatic TvDbId resolution via TMDb external_ids (FanartTv's TV API requires TvDbIds)
• Custom TV Scraper integration — FanartTv is now available as an option for image fields (Banner, Poster, Fanart, Thumb, SeasonPoster, SeasonBanner, SeasonThumb, ExtraArts) in the Custom TV Scraper settings, with full dispatch wiring so selections actually trigger FanartTv loading
• Fallback checkbox — New "Use as fallback for missing images" setting in FanartTv configuration. When enabled, FanartTv automatically fills in any missing images after scraping, regardless of which scraper is configured. Works for both movies and TV shows.
• Movie ID resolution — Movies without a TMDb/IMDB ID (e.g. scraped via VideoBuster) now get their TMDb ID resolved automatically via title search before querying FanartTv

Fixes #1982

Limitations

• FanartTv does not provide season backdrops or extra fanarts for TV shows — these fields are not offered in the Custom TV Scraper dropdowns
• FanartTv's TV API only accepts TvDbIds; shows without a TvDbId mapping in TMDb will appear in search results but return no artwork

Test plan

Automated:

• Unit tests passing (688 assertions)
• Integration tests passing
• Scraper tests: same results as master (21 pre-existing failures)
• Pre-push checks: clang-format + quick_checks all passing

Manual — Movies (VideoBuster single scraper):

• Fallback off → only scraper images, rest empty
• Fallback on → all FanartTv images filled (Logo, ClearArt, CdArt, Banner, Thumb)
• TMDb ID auto-resolved when not available (verified in logs)
• Manual image dialog: FanartTv available for all fields

Manual — TV Shows (TMDb TV single scraper, "Travelers"):

• Fallback off → only TMDb images (Poster, Fanart)
• Fallback on → Logo, ClearArt, CharacterArt, Banner, Thumb filled automatically
• Season banner/poster/thumb from FanartTv
• Episodes correctly skip FanartTv (no episode-level images in API)
• FanartTv search via TMDb working in manual image dialog

Manual — Settings:

• Fallback checkbox visible, German translation, tooltip working
• Setting persists across restart
• Custom TV Scraper dropdowns show FanartTv for image fields

Developed with AI assistance (Claude Code / Opus 4.6).

[kolbdog323]

You shouldn't remove tvdb all together since not all banners are on fanart.tv they only exist on tvdb same with season posters

[Arny80Hexa]

Hi @kolbdog323,

Thanks for the feedback! To clarify: this PR does not remove TheTvDb. It only changes how FanartTv's internal TV show search works — from using TheTvDb to TMDb for finding shows and resolving TvDb IDs.

TheTvDb Images is still available as an image provider in the manual image dialog, so you can still browse and pick banners/season posters from TheTvDb there.

The deprecation of TheTvDb as a metadata scraper was a separate decision by the maintainer (see #1120) and is not part of this PR.

[kolbdog323]

Hi @kolbdog323,

Thanks for the feedback! To clarify: this PR does not remove TheTvDb. It only changes how FanartTv's internal TV show search works — from using TheTvDb to TMDb for finding shows and resolving TvDb IDs.

TheTvDb Images is still available as an image provider in the manual image dialog, so you can still browse and pick banners/season posters from TheTvDb there.

The deprecation of TheTvDb as a metadata scraper was a separate decision by the maintainer (see #1120) and is not part of this PR.

Sorry about that it just sounded like it the way it worded

[Arny80Hexa]

No worries! Your concern is absolutely valid — as a user, you don't want features to break after an update. That's exactly the kind of feedback that keeps PRs honest. In this case nothing is removed, only added, but it's good that you flagged it.