Add optimistic locking based on versions

[solenuk]

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

Assumptions & Constraints

• Uses the standard JPA version field.
• This is purely a concurrency safeguard, it doesn't change any business rule or existing field.

Preconditions

1. TaskAssignment entity and its migration already exist.

Business rules

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

Acceptance Criteria

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

Out of scope

• Any UI handling for a version conflict.
• Adding versioning to any entity other than TaskAssignment.

Epic link
Epic #385 - EPIC: Tasks linkage to tours

Tasks

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