scratch: the scratchpad is the inbox, with ways out and a search

A note now leaves the scratchpad by becoming something else: `K` keeps it
as a document, as before; `C` makes it a card where `:card` would put one,
the first line its title; `A` opens `:append` with the document on screen
filled in, Tab completing any other title, and puts the note at its end.

`/api/capture` takes `to: "note"` and `to: "document"` (named as a link
names it), and `note`, the scratch note the text came from, dropped once
the text has landed. Appending keeps the revision before it and indexes
links and media, as a save does.

Migration 0028 adds `scratch_fts`; `GET /api/scratch?q=` searches it, and
`/` lists note hits under "notes" after the cards, Enter lowering the
drawer at the note.

The `c` capture line cycles its target with Tab: a card, a note, or a
timed line in today's entry, which `:today <text>` now writes.

docs/wiki/Files-and-Search.md

@@ -59,6 +59,8 @@ previous result beside the list, and `:noh` clears the search.
 Cards are searched too, by their title, description and checklist steps. They
 are listed after the documents, under `cards`, with the board and list each is
 on. `Enter` on one turns it over on its board; `n` and `N` only light it up.
+Notes on the scratchpad come last, under `notes`, whatever space is searched;
+`Enter` lowers the scratchpad at the note.
 
 ![A search that found a line in a log file](../images/search.png)