cards: a document and its cards, seen from both ends

A document now lists the cards about it: the ones whose notes it is and
the ones whose description links to it. GET /nodes/{id}/cards answers
that, and the node itself carries it as `cards`. A line under the
document's meta line says what is still open and when it is first due;
it and `gc` open a new cards part of the info sheet, where j/k picks a
card, Space, H/L, D, a and `:` act on it, and Enter turns it over on its
board. `c` while reading a document makes a card about it.

Card verbs act on the card in focus wherever it is looked at: the
board's cursor, the card turned over, the agenda's highlight, the card
picked in the info sheet, or a document's only open card. patchCard,
sendCard and toggleDone take that card, so patchAgendaCard,
sendAgendaCard and finishCard are gone, and :due, :repeat, :label and
:done no longer refuse to run away from the board.

Migration 0027 adds card_links, links for a card's description: indexed
on every write, backfilled on start, followed and rewritten on a rename
or move like a document's, and claimed when a title they named appears.
It also adds cards_fts over title, description and checklist steps;
GET /cards?q= searches it, and / lists the cards found after the nodes,
Enter turning one over on its board.

docs/wiki/Files-and-Search.md

@@ -56,6 +56,10 @@ folder it is in. `Tab` or `Ctrl-N` / `Ctrl-P` walk them without leaving the
 line, and `Enter` opens the one lit. Afterwards `n` and `N` open the next and
 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.
+
 ![A search that found a line in a log file](../images/search.png)
 
 Each word you type matches the start of a word, so `inge` finds "ingest". All