fix: add Notebook.notes relation to MCP schema

Tools query _count.notes on Notebook which requires the notes relation.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

mcp-server/prisma/schema.prisma

@@ -64,6 +64,7 @@ model Notebook {
   userId    String
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
+  notes     Note[]
   labels    Label[]
 }