Add optimistic locking based on versions

[solenuk]

As a developer I want to add optimistic versioning to TaskBody so that two concurrent updates to the same task can't silently overwrite each other.

Assumptions & Constraints

• Uses the standard JPA version field, a long column that increments on every update.
• This is purely a concurrency safeguard, it doesn't change any business rule or existing field.
• A concurrent update conflict should surface as a clear error to the caller.

Preconditions

1. TaskBody entity and its migration already exist.

Business rules

1. Every update to a TaskBody row increments its version.
2. An update based on a stale version is rejected, not silently applied.
3. Creating a new TaskBody starts at the initial version, no special handling needed there.

Acceptance Criteria

• task_bodies has a version column, not null, defaulting to 0.
• The entity has a version field mapped to that column.
• Updating a TaskBody with a stale version results in a clear, mapped error response.
• A normal, non concurrent update still works exactly as before.

Out of scope

• Any UI handling for a version conflict, that's a separate concern once this exists.
• Adding versioning to any entity other than TaskBody.

Epic link
Epic #6 - EPIC: Tasks

Tasks

• Add the version column migration for task_bodies.
• Add the version field to TaskBody.
• Confirm a normal update still works and a stale update is rejected.