search: a file is found by what it says

The text of every file in the tree that is text, by its type or by its
first 8 KiB reading as UTF-8 without a NUL, now goes into `file_text` and a
second FTS5 index beside the documents', up to its first 256 KiB, the same
part the viewer shows. A search asks both, lists a node found in both once,
and marks a file found by its text with `in_text`; the list shows the words
around the match under its name.

There is one way in, `file_text::catch_up`, which reads every unsealed file
that has no row yet. An upload, an import, an SFTP save and a decrypt call
it once their own transaction is done, and the server runs it at start for
anything that slipped past. A file that is not text gets an empty row, so it
is read once and not again.

The triggers keep it honest the other way. New bytes drop the old text, so
an SFTP save is read again, and `:encrypt` writes a file under a new media
id, so its plaintext is gone in the same statement that seals it. The text
of a sealed file is refused outright, whatever path tries to store it;
otherwise it would sit in `diary.db` and its mirror beside the sealed bytes.

PDFs and office documents are not read: their text needs a parser this
binary does not carry.

web/src/lib/Help.svelte

@@ -39,7 +39,7 @@
     ['b', "open this space's board"],
     ['c', "capture a card into this space's inbox, without leaving here"],
     ['Esc · q', 'close the document, and its tile when there are several'],
-    ['/', 'full-text search, inside the current space — Tab or Ctrl-N/P walk the results, Enter opens one'],
+    ['/', 'full-text search, inside the current space and its text files — Tab or Ctrl-N/P walk the results, Enter opens one'],
     ['n / N', 'next / previous search result, opened beside the results — :noh clears the search'],
     ['s', 'toggle sharing for the document — asks before taking it back'],
     ['y', 'mint a new share link and copy it — asks, the old one stops working'],