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

src/thoughtsync/models/all.py

@@ -3,4 +3,15 @@
 Imported for side effects only (model registration on Base.metadata).
 """
 
-from . import group, label, note, note_attachment, note_item, note_link, settings, share, user  # noqa: F401
+from . import (  # noqa: F401
+    group,
+    label,
+    note,
+    note_attachment,
+    note_item,
+    note_link,
+    note_revision,
+    settings,
+    share,
+    user,
+)