[Feature] - OPML Overwrite vs Append Import Toggle

**Is your feature request related to a problem? Please describe.**
Does the import process overwrite the existing rss subs or does it append them to the existing ones? I tried to do an import to transform a broken nitter rss instance to a new one after find and replacing an export with the new URL, and it seemed to just add the new subs without removing the broken subs that I had deleted from the OPML file.

**Describe the solution you’d like**
Unless I'm mistaken, and I somehow did this wrong, would it be possible to add a toggle for whether to overwrite versus just appending to the existing subscription list? I would imagine that you would want to make the default appending though since that is less destructive?

**Describe alternatives you’ve considered**
Not really sure if there is another good alternative to an append/overwrite option.

**Additional context**
N/A

Start at FreshRSS's OPML import flow and trace both the import handling and the UI where an import option could be presented. Define the append behavior as the safe default, add an overwrite choice that removes subscriptions absent from the imported OPML, and verify both modes preserve the intended subscription list.

Written by the indexing model from the issue text.