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

[ArchILLtect]

Problem / context

Avoid orphan rows and unintended cascades as submissions are created and items deleted.

Proposed solution

Audit mappings and adjust fetch/cascade where sensible; add 1–2 DAO tests if changes are made

In scope / Out of scope

In scope

• Mapping review and small tests (only if changes are made)
  Out of scope
• Broad refactors or performance tuning

Acceptance criteria

• No unexpected orphans in basic delete paths; mappings remain sane

Area

area:persistence

Dependencies / related issues

Issue #27--Related to Drill submission flow

Checklist

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