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/API.md

@@ -33,6 +33,7 @@ sealed or not.
 | `POST` | `/api/lists/{id}/cards` | add a card |
 | `PUT`/`DELETE` | `/api/cards/{id}` | update (title, body, due, repeat, checklist, labels, done, link) / delete; finishing a repeating card answers its copy as `again` |
 | `POST` | `/api/cards/{id}/move` | to another list and place in it; `again` as above |
+| `GET` | `/api/cards?q=…` | cards whose title, summary or a checklist step matches, open ones first; `space` keeps it to one board |
 | `GET`/`POST` | `/api/scratch` | the scratchpad's notes, the one touched last first / a new note `{ body }` |
 | `PUT`/`DELETE` | `/api/scratch/{id}` | rewrite a note `{ body }` / drop it |
 | `POST` | `/api/nodes/{id}/move` | reparent, and place among the new siblings |
@@ -43,6 +44,7 @@ sealed or not.
 | `GET` | `/api/nodes/{id}/revisions` | a document's earlier versions, the newest first |
 | `GET` | `/api/nodes/{id}/revisions/{rev}` | one earlier version, with its text |
 | `GET` | `/api/nodes/{id}/backlinks` | the documents that link to this one, with the line each does it on |
+| `GET` | `/api/nodes/{id}/cards` | the cards about this document: its own (`node_id`) and the ones whose summary links here — also in the node itself as `cards` |
 | `POST`/`DELETE` | `/api/nodes/{id}/share` | mint or rotate / revoke a share link |
 | `GET` | `/api/export` | the whole workspace, boards and dates included, as a zip |
 | `GET`/`POST` | `/api/backup` | Proton Drive mirror: status / run now |