Fix race condition in WorkRelation ordinal assignment using advisory locks

[ja573]

Resolve a race condition in WorkRelation::create() where concurrent insertions could compute the same relation_ordinal for inverse relations, triggering duplicate key violations on work_relation_ordinal_type_uniq.

The fix includes:

• Moving max(relation_ordinal) selection fully inside the transaction
• Introducing a pg_advisory_xact_lock keyed on (related_work_id, inverse relation type) to serialise ordinal assignment for each work/type pair