CHW-8329 MonitoredTargetDuplicate

Default message: a monitored target already exists for this repository label and branch; targets are unique per organisation, repository label and branch — update the existing target, or use a different repository label if these are genuinely two branches
HTTP status: 409 (client)

Problem. Creating a target returned 409. Cause. A target already exists for this (organisation, repo_label, branch) triple, which is the uniqueness key. Fix. Update the existing target, or use a different repository label if these really are two different branches. Note that the upload path upserts by design — re-uploading an inventory for an existing target replaces its package set rather than conflicting, so this code is raised only by the explicit-create route. HTTP 409. Fields: repo_label, branch, existing_target_id.