Enable users to read their own roles

Allow users to query and read the roles assigned to themselves through the RBAC system.

Scope:

1. Creator/Purger pattern for user_roles → association_scopes_entities

2. my_roles GraphQL query

3. search_roles() RBAC visibility update

4. DataLoader for role queries

5. Backfill migration for existing user_roles data

Reference:

- BEP-1048: RBAC Entity Relationship Model

JIRA Issue: BA-4810