* Make sure that `img_id`, `audio_id` and `cover_id` fields are always available.
The values are set '' where they are not applicable.
Having `img_id` is necessary for the default `archive_fmt`, the other fields are handled for consistency.
* Allow downloading more than one cover.
The previous behavior is kept as-is, but setting the "covers" option to "all" now grabs all available covers.
* Add support for downloading subtitles
Allows filtering subtitles by source type (ASR, MT) and language.
* Ensure archive uniqueness for covers and subtitles.
* Update the URL test pattern to include the `image` extension.
Although Tiktok may serve the covers with jpeg content, the file ending can be `.image`.
The test before 0c14b164 failed because the asserted URL did not match all cover types, but the now used pattern needs the mentioned file ending.
* Add support for "creator_caption" subtitles in "LC" format.
These subtitles have the keys "Format" set to "creator_caption" and "Source" to "LC".
* Add "LC" (Local Captions) as a subtitle source type in the documentation
* Code deduplication and renaming subtitle metadata
Changed the item type from singular `subtitle` to `subtitles`.
Removed the wrong descriptor `cover` from the subtitles fallback title.
* Refactor subtitle filtering
The filter is now prepared in `_init` to prevent parsing the same config parameter for every item.
The `_extract_subtitles` function will still extract if either filter (source or language) matches.
* Generate a `file_id` for subtitles
Subtitles have multiple fields that determine the unique file, so these are simply concatenated.
This is similar to the cover types, only with more variations.
* Added tests for subtitles
* fix docs entries
* fix '"covers": "all"'
* simplify some code
* Fix fallback title for subtitles
Added the missing "f" to the f-string and added "subtitle" to the title.
The resulting title will look like "TikTok video subtitle #1234567"
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