Assign ISBN to an epub export profile
Due to https://gitlab.coko.foundation/ketida/ketida/-/issues/770 and https://gitlab.coko.foundation/ketida/ketida/-/issues/802 we have multiple ISBNs in metadata, and all of those ISBNs appear on copyright pages. This is allowed for most ISBN rules including Lulu's.
In the epub content.opf file we have multiple ISBNs too. For example if you have two ISBNs in metadata, one with the label 'Hard cover' and the other with label 'Epub', the content.opf will include:
<dc:identifier id="BookId-Hard-cover">urn:isbn:978-1-23-456789-0</dc:identifier>
<dc:identifier id="BookId-Epub">urn:isbn:978-1-23-456789-1 </dc:identifier>
The content.opf of the epub also specifies a Unique Identifier. We are currently using the first ISBN in the list as the unique-identifier value. Therefore, at present, in some cases the unique-identifier will be an ISBN for that book that is not the ISBN that the user has labelled for that epub.
However Apple and Amazon will reject the content if the epub ISBN the user has provided to them does not match the unique-identifier ISBN in the content.opf, so further development is required.
We want users to be allowed more than one epub export profile, and each might have (but don't necessarily have) unique ISBNs, therefore we would have to tie an ISBN to an epub export profile instead of having one epub ISBN checkbox in metadata. For example a dropdown list of available ISBNs (drawn from metadata) shows on the export profile sidebar for epubs. When the epub is generated upon the Download action, the selected ISBN of the export profile is used as the unique identifier. The other ISBNs for the book can still appear in the ISBNs list of the content.opf as well as the interior copyright page.