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,
+)

src/thoughtsync/models/note_revision.py

@@ -0,0 +1,27 @@
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, Index, Text, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from . import Base
+
+
+class NoteRevision(Base):
+    """A point-in-time snapshot of a note's title+body, written on each edit that
+    changes either — so an accidental overwrite can be viewed and restored. Only
+    title+body are versioned in v1 (not items/attachments/labels)."""
+
+    __tablename__ = "note_revisions"
+    __table_args__ = (Index("ix_note_revisions_note_created", "note_id", "created_at"),)
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    note_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("notes.id", ondelete="CASCADE"), nullable=False
+    )
+    title: Mapped[str | None] = mapped_column(Text(), nullable=True)
+    body: Mapped[str] = mapped_column(Text(), nullable=False, server_default="")
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())