feat: MVP-4 Sprint 3 - Version history

- Add NoteVersion model for storing note snapshots
- Add versions API (list, create, get, restore)
- Add version history UI dialog in note detail page
- Create version snapshot before each note update

prisma/schema.prisma

@@ -78,3 +78,13 @@ model NoteCoUsage {
   @@index([fromNoteId])
   @@index([toNoteId])
 }
+
+model NoteVersion {
+  id        String   @id @default(cuid())
+  noteId    String
+  title     String
+  content   String
+  createdAt DateTime @default(now())
+
+  @@index([noteId, createdAt])
+}