Sync watchlist with MyAnimelist and AniList

[zoriya]

Feature description

Continuation of #347

[zoriya]

To know which episode to update on mal/anilist, something like https://github.com/Anime-Lists/anime-lists should be used when scanning. The only issue with this list is that it's based around tvdb mapping, but kyoo uses tmdb.

I have no idea how to map from tvdb and tmdb ; maybe just base off absolute ordering? I think that's what is currently used in thexem.info mapper.

Episode should store their external ids as

{
   "externalIds": {
       "anidb": {"showId": "123", "season": null, "episode": 1},
       "tvdb": {"showId": "852", "season": 1, "episode": 13}
   }
}

because episodes ids on most websites are not considered stable (if they exists at all).

[zoriya]

maybe use thexem as the primary mapping?

[zoriya]

or parse anime with anidb directly in the scanner?
They don't have a search api but they have exports of all names so we could search locally. Complex season layout (like monogatari) would be handled using thexem. More standard seasons could be looked up using the relations tag? Not sure how to handle OVA or side stories with that

[zoriya]

Current plan:

• Search on AniList and TMDB and pick best match out of the two (aka: figure out if it's an anime or not, searching batman on anilist gives an anime named baton)
• Find anidb id from anilist id using https://github.com/manami-project/anime-offline-database/
• Use thexem to find season-number and parent show
• If thexem lookup failed, walk the "relations" graph of anilist to manually calculate the season number.
• Use thexem to get tvdb id from show and fetch tmdb metadata using the "Find by Id" method and merge metadata between those two.
• If thexem lookup fail ???. Maybe https://github.com/Anime-Lists/anime-lists/ ?

PS: it seems thexem is wrong in some cases, for example for monogatar it is wrong on anidb episodes count. This is not the case in the anime-list repo.