Delete Placeholder

Remove a placeholder the current user owns.

Cascades: circle_members, placeholder_claim_links, relationship_link_requests (if table present), AND relationship_edges rows referencing this placeholder via subject_placeholder_id or related_placeholder_id.

Refuses (409) if the placeholder has already been claimed — once a real user has resolved into the placeholder, the deletion no longer makes sense; the owner should unfriend the claimed user instead.

Chapter C4 (May 2026 — Cross-Cutting C4): merged from the previously shadowed handler in routes/social_supplements.py:55. That handler was the correct implementation (it cascaded relationship_edges and refused on claim) but it was never reached because this same path was mounted earlier on /api/social. The shadowed handler has been deleted as part of this chapter.

Distinguishes 404 (not found) from 403 (found but owned by someone else) explicitly. The previous combined-condition WHERE clause returned 404 in both cases, which obscured permission errors during debugging.