fix: scope graph entities by user

[wayyoungboy]

Summary

Closes #1002.

This PR scopes OceanBase graph entities by user_id so graph entity deduplication and seed discovery no longer reuse nodes across users.

Changes:

• adds user_id to graph_entities and indexes (user_id, name)
• writes user_id when creating graph entities
• filters graph entity vector search by user_id
• filters entity name lookups used by relationship deletion by user_id
• recreates legacy unscoped graph tables when an existing graph_entities table lacks user_id
• avoids dropping tables if schema inspection fails; startup raises instead
• adds unit coverage for schema creation, legacy migration, entity creation, search scoping, and deletion lookup scoping

Validation

• python -m pytest tests/unit/test_oceanbase_graph.py -q
• python -m pytest tests/unit -q
• python -m flake8 src/powermem/storage/oceanbase/oceanbase_graph.py tests/unit/test_oceanbase_graph.py --select=F601,F821,E999
• git diff --check