feat(memory): entity canonicalization with alias table

## Summary

Follow-up to graph memory epic (#1222). The UNIQUE(name, entity_type) constraint cannot represent two semantically different entities with the same name and type.

Add:
- `canonical_name` column on `graph_entities`
- `graph_entity_aliases` table (entity_id, alias_name) for variant names
- Resolution checks aliases before creating new entity

**Depends on**: #1225 (extraction pipeline)