feat: add listed field to note for public listing visibility

Notes with listed=false are filtered out from all GET /notes queries.
The field defaults to true so existing and new notes without it remain visible.
Migration handles existing databases with ALTER TABLE ADD COLUMN.

lexicons/space/remanso/note.json

@@ -46,6 +46,10 @@
             "type": "string",
             "description": "Display theme for the note.",
             "knownValues": ["light", "dark"]
+          },
+          "listed": {
+            "type": "boolean",
+            "description": "Whether the note appears in public listings. Defaults to true."
           }
         }
       }