wopi: onlyoffice support

collabora online (cool/code/coolwsd) seems to work thanks to #1495 however editing with onlyoffice is currently not possible

https://github.com/9001/copyparty/pull/1495#issuecomment-4959163573 mentioned they were able to open a document read-only, but I'm not even getting to that point; when I try to create a new document with the "new textfile" feature in the copyparty ui, for example with the filename `some.docx` then I see onlyoffice successfully initializes the empty document in the copyparty volume, then onlyoffice asks for the properties of the document, and then nothing more happens; onlyoffice never asks for the contents of the document, and the onlyoffice ui is stuck in a "loading" state, as if the document has not yet been transferred into the editor, however there is zero explanation why in either the copyparty or onlyoffice logs

perhaps onlyoffice doesn't like something about the wopi response sent from copyparty?

a complicating factor is that this whole feature (the wopi integration) is something i do not care for, something i will never use, so I do not have the motivation to dig into this further. I am also suspecting I have not configured onlyoffice correctly, so maybe it already works just fine?

perhaps someone with a working onlyoffice installation and a motivation to look into this could give it a shot

この issue は OnlyOffice との WOPI 統合に関するものです。まず、copyparty の WOPI 実装を調べます。おそらく wopi.py や handlers に関連するファイルが対象になります。Collabora に送信されるレスポンス（こちらは動作します）と、OnlyOffice に送信されるレスポンスを比較します。copyparty と OnlyOffice の両方のログを確認し、エラーがないか調べます。目的は、OnlyOffice がドキュメントのプロパティを要求した後に停止し、コンテンツを一度も要求しない理由を確認することです。WOPI エンドポイントと JSON レスポンスを OnlyOffice のドキュメントに照らして検証します。

索引モデルが issue の本文から書いたものです。