Add storage

cyynote-backend/target/classes/db/migrations/V4__backfill_note_userid.sql

@@ -0,0 +1,5 @@
+-- V4: Assign existing notes without an owner to userid=1 (first admin)
+-- This ensures production data created before multi-user isolation is not lost.
+UPDATE note SET userid = 1 WHERE userid IS NULL OR userid = 0;
+
+INSERT OR IGNORE INTO db_version (version, applied_at) VALUES (4, datetime('now'));