feat(persistence): Finalize User entity + DAO (minimal OIDC linkage)

[ArchILLtect]

Problem / context

User model needs to be present/confirmed for auth features and submission ownership. Confirm mappings.

Proposed solution

Create/confirm User entity (id, username/email, roles/authorities minimal), DAO, and relationships to Submission (if required now).

In scope / Out of scope

In scope

• Entity, DAO, basic CRUD tests, minimal fields for OIDC principal linkage.

Out of scope

• Full RBAC persistence and profile pages (later weeks).

Acceptance criteria

• User DAO CRUD tests pass
• Relationships compile and basic lookups work (e.g., find submissions by user)

Area

area:persistence

Dependencies / related issues

Cognito login; may map OIDC subject to a local user in future work

Checklist

• I’ve checked existing issues for duplicates.
• I can help implement this and open a PR.