M6: version history — note revisions + restore

A note's title+body is snapshotted on each edit that changes either, so an accidental overwrite can be viewed and restored (task 1906). Underwrites 'dump freely, nothing is lost'.

Backend: note_revisions table (migration 0014) + NoteRevision model; update_note records a revision of the PRE-edit state whenever title/body changes; GET /api/notes/<id>/revisions (newest 50) and POST /api/notes/<id>/revisions/<rev_id>/restore (snapshots the current state first so restore is itself undoable, then applies the revision with the usual title/body ripple — display name, links, #tags, backlinks). Title+body only in v1.

Frontend: a History toggle in the modal editor opens a panel of past versions (timestamp + preview) with per-row Restore. Store gains fetchRevisions/restoreRevision.

Migration 0014 runs on deploy; DB behavior operator-verified (no Postgres CI lane).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/Icon.vue

@@ -22,6 +22,7 @@ const paths: Record<string, string> = {
   calendar: '<path d="M8 2v4"/><path d="M16 2v4"/><rect width="18" height="18" x="3" y="4" rx="2"/><path d="M3 10h18"/>',
   close: '<path d="M18 6 6 18"/><path d="m6 6 12 12"/>',
   merge: '<circle cx="18" cy="18" r="3"/><circle cx="6" cy="6" r="3"/><path d="M6 21V9a9 9 0 0 0 9 9"/>',
+  history: '<path d="M3 12a9 9 0 1 0 9-9 9.75 9.75 0 0 0-6.74 2.74L3 8"/><path d="M3 3v5h5"/><path d="M12 7v5l4 2"/>',
 };
 </script>