How do we create hyperlinks both to other `items` and `external` content?

What I'm trying to figure out is: how do we link two items?
How do we create a _hyperlink_ to external content?
These might be _similar_ things or _entirely_ different challenges.

Provisionally I'm noting down the following:
+ `link` - a hyperlink
+ `id`: `Int`
+ `url`: `String`
+ `item_id` (FK item.id)

But this is by no means the "solution"!
Help very much welcome/wanted!

The issue names no files, tests, or entry points. Start by locating how items are represented and persisted, then clarify whether links between items and links to external content should share a model. Done means the relationship and URL design are agreed and documented well enough to implement.

Written by the indexing model from the issue text.