Fix: Integration with Pinboard: Preserve Existing Bookmarks

[ztec]

The Issue:
When saving an entry that is already bookmarked on Pinboard,
Miniflux was overriding all existing data on Pinboard. This action
removed any extended content or, worse, changed the private settings
to public, making previously private bookmarks publicly available.

The Fix:
Now, upon saving an entry as a bookmark, I first fetch it. If it
already exists, I apply the necessary modifications (adding tags and any state)
that Miniflux would have normally done, then add it again. This way, no
data is lost in the process. Pinboard has a stable API, so I don't anticipate
any new fields being added soon.

I manually tested the integration by hitting the save button in the following situations:

• Entry URL does not exist on Pinboard:
  • Bookmark is properly added on Pinboard with tags and "to read" status according to Miniflux settings.
• Entry URL already exists on Pinboard:
  • Existing data remains unchanged.
  • Tags from Miniflux settings are properly added to the bookmark.
  • "To read" status is set to yes when the option is checked in Miniflux. Nothing is changed otherwise.

Do you follow the guidelines?

• I have tested my changes
• There is no breaking changes
• I really tested my changes and there is no regression
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• I read this document: https://miniflux.app/faq.html#pull-request