feat(db): add users, roles, and source access mappings

[alucero270]

feat(db): add users, roles, and source access mappings

Summary

Extend persistence to model users, roles, and source-scoped access control relationships.

Scope

• Files: schema and migration files, persistence layer, data-model docs as needed
• Components: users, roles, access mappings, source-scoped authorization data

Acceptance Criteria

• the data model can represent users and roles
• the data model can represent source-scoped access mappings
• the updated model is documented clearly enough to test and verify

Notes

• This is authorization data-model work, not UI work