feat(persistence): Enforce one DrillItem per Challenge; handle concurrency and race conditions

[ArchILLtect]

Problem / context

Under concurrent load, duplicate DrillItem rows could be created for the same Challenge, and concurrent updates might overwrite streak/nextDueAt values (lost updates).

Proposed solution

Uniqueness & race handling

• Enforce one DrillItem per Challenge via a unique constraint (drill_items.challenge_id).
• Update getOrCreateDrillItem to catch DataIntegrityViolationException and re-fetch the existing record.

Optimistic locking

• Utilize the @Version field added in MVP.

Tests

• Add DAO/service concurrency tests for create/update race scenarios.

Index

• Add unique index on drill_items.challenge_id in Flyway/Liquibase migration.

In scope / Out of scope

In Scope

• Service/DAO concurrency handling and tests.
• Schema constraint via migration (Week 8 milestone).

Out of Scope

• Refactoring unrelated domain entities.
• Non-persistence optimizations.

Acceptance criteria

• Unique constraint prevents duplicate DrillItem per Challenge.
• Concurrent updates use optimistic locking (@Version).
• Concurrency tests confirm correctness and stability.
• All existing tests remain green.

Area

area:persistence

Dependencies / related issues

No response

Checklist

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