feat(faces): PeopleService — identity clustering + use cases

Phase 2 increment 4:
- recluster(user): union-find connected-components over the user's face
  embeddings (cosine threshold); groups of >= min_faces become persons, and
  an existing person's name is preserved across reclusters. O(n^2) — fine
  for moderate libraries; an ANN index is the documented scale-up.
- caller_id-scoped use cases for the HTTP layer: list_people (non-empty
  clusters, cover thumbnail, most-photographed first), person_photos,
  faces_for_file, rename, hide, merge, and delete_all (right to erasure).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JW6ghFMDtnRYuYNzZhb47M

src/application/services/mod.rs

@@ -20,6 +20,7 @@ pub mod magic_link_invite_service;
 pub mod music_service;
 pub mod nextcloud_file_id_service;
 pub mod nextcloud_login_flow_service;
+pub mod people_service;
 pub mod places_service;
 pub mod recent_service;
 pub mod recipient_notification_service;