feat: mount spaces/folders over SFTP with SSH-key auth

Add an in-process SFTP server (russh) backed directly by the nodes
table, so a space can be edited from nvim or synced with sshfs without
a separate filesystem mirror. Access is authenticated by public key
against a new ssh_keys table, managed via :sshkey in the app; the
feature stays off until WORKSPACE_SFTP_BIND is set.

migrations/0009_ssh_keys.sql

@@ -0,0 +1,13 @@
+-- Public keys allowed to mount the workspace over SFTP. A single-user
+-- application still wants more than one key: one per machine that mounts it,
+-- so losing a laptop means revoking one row rather than a shared secret.
+CREATE TABLE ssh_keys (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    name        TEXT    NOT NULL,
+    -- `ssh-ed25519 AAAA...`, comment stripped: the comment is not part of the
+    -- key and would otherwise make the same key look different twice.
+    public_key  TEXT    NOT NULL UNIQUE,
+    fingerprint TEXT    NOT NULL UNIQUE,
+    created_at  INTEGER NOT NULL,
+    last_used_at INTEGER
+);