links: a rename or a move no longer breaks them

A document links to another by its path, `[the plan](/n/work/plan)`, or
now by its title, `[[Plan]]` or `[[Plan|the plan]]`. What each document
links to is kept in a new `links` table, rebuilt whenever its text is
written: from the browser, over the mount, or by an import. A title is
looked for in the linking document's space first, then in every other.

Renaming or moving a document, or a folder above it, rewrites the links
to it in every document that has one, in the same transaction: the path
to the new one, a title to the new name, with the label left as it was.
Each rewritten document keeps a revision of what it said before. A link
to something in the trash keeps its row, so it follows the thing when it
is restored somewhere else. A document created, or renamed, under a title
that a link was waiting for is found by that link from then on.

The reader draws `[[titles]]` as links, and a title no document has yet
as a dimmed one that offers to write that document. `gf` follows either
from the editor, and `[[` there completes titles from the search. `gb`
and `:backlinks` list the documents that link to the open one, with the
line each does it on; the line under the title counts them. A share page
shows these links as plain text, as it does every /n/ link.

The documents written before this are indexed once, when the server
starts and finds the table empty.

migrations/0021_links.sql

@@ -0,0 +1,17 @@
+-- The links between documents, as their text writes them: a `/n/...` path,
+-- or a `[[title]]`. Rebuilt from a document's text whenever it is written,
+-- and read the other way round: for what links to a document, and for the
+-- texts to rewrite when what they link to is renamed or moved.
+--
+-- Nothing fills it for the documents already written; the server indexes
+-- them all when it starts and finds it empty (see `links::backfill`).
+CREATE TABLE links (
+    from_id INTEGER NOT NULL REFERENCES nodes (id) ON DELETE CASCADE,
+    kind    TEXT    NOT NULL CHECK (kind IN ('path', 'wiki')),
+    -- What the link says: the path after `/n/`, or the title in the brackets.
+    target  TEXT    NOT NULL,
+    to_id   INTEGER NOT NULL REFERENCES nodes (id) ON DELETE CASCADE,
+    PRIMARY KEY (from_id, kind, target)
+) WITHOUT ROWID;
+
+CREATE INDEX idx_links_to ON links (to_id);