v1.7.2 — delete solo event clips on identity delete

Refines the recursive-delete policy (from 1.7.1) per the chosen rule: delete the event clip only if the person was the sole subject.

When you delete an identity, any event where it was the only associated identity is now deleted along with its clip + thumbnail files. Events that also involve other people are preserved (shared footage) — only the deleted person's link is nulled. Association is computed across both the denormalized Event.identity_id and the event's sightings, and applied per delete-set so single / bulk / clear-all all behave correctly.

Verified live: deleting an identity removed its 8 solo events + clips, while a (synthetically) shared event survived with its clip intact and the surviving identity untouched.