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

alembic/versions/0014_note_revisions.py

@@ -0,0 +1,34 @@
+"""note_revisions
+
+Revision ID: 0014
+Revises: 0013
+Create Date: 2026-07-22
+
+Version history: a snapshot of a note's title+body written on each edit that
+changes either, so an accidental overwrite can be restored.
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import UUID
+
+revision = "0014"
+down_revision = "0013"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "note_revisions",
+        sa.Column("id", UUID(as_uuid=True), primary_key=True),
+        sa.Column("note_id", UUID(as_uuid=True), sa.ForeignKey("notes.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("title", sa.Text(), nullable=True),
+        sa.Column("body", sa.Text(), nullable=False, server_default=""),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False, server_default=sa.func.now()),
+    )
+    op.create_index("ix_note_revisions_note_created", "note_revisions", ["note_id", "created_at"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_note_revisions_note_created", table_name="note_revisions")
+    op.drop_table("note_revisions")