Add TMDB as provider for episodes

[ChriRas]

Adds TMDB as a third provider option for episode/TV-show metadata, alongside the existing TVDB and TVmaze providers. Closes #277.

Key Changes

• You can now pick TMDB when looking up episode metadata, not just movies.
• Set it via --episode-api tmdb on the CLI or episode_api: tmdb in your config file.
• Episodes can be matched by TMDB ID, IMDb ID, or by series name plus season/episode (or air date).
• Existing TMDB API key (api_key_tmdb) is reused — no extra configuration needed.
• Movie lookups via TMDB are unchanged; both modes coexist cleanly.

Notes

• Implements the request from Support TMDB as an episode API too. #277: TMDB's TV catalog has grown to the point where many series are better-curated there than on TVDB, so users now have the choice.
• Adds tests for the new endpoints, the new provider, the expanded episode_api choices, and the updated provider cache.

[berglh]

Hi @ChriRas, I tried your build and it's working great for me. Thanks for the PR, hopefully @jkwill87 will merge after testing.