chore(persistence): Review lazy/eager and cascade settings for core entities

[ArchILLtect]

Problem / context

We need to confirm fetch/cascade settings to avoid N+1 or orphan records during submissions and deletes.

Proposed solution

Review entities and annotate fetch/cascade explicitly where needed (e.g., submissions under challenge; drillItem relationships).

In scope / Out of scope

In scope

• Audit mappings and adjust fetch/cascade; add a couple of DAO tests to assert cascades.

Out of scope

• Performance tuning beyond fetch mode sanity.

Acceptance criteria

• No unexpected orphan rows on delete
• Reasonable defaults for fetch; no major N+1 in common paths

Area

area:persistence

Dependencies / related issues

Test Coverage issue for DAO behaviors if updated

Checklist

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