ragleap-graph v0.6.5

Adds user_id= for per-user data isolation across upsert_document() and all read methods, matching the existing namespace= pattern. Includes backfill_user_id_defaults() as a one-time migration for pre-upgrade data. Read methods use coalesce() so user_id=None correctly matches legacy data without requiring migration first. Two real bugs caught and fixed before release. Live-verified against real Neo4j. Full suite: 86 passed, 1 skipped. Published to PyPI.