feat(faces): indexing pipeline + DI wiring

Phase 2 increment 5:
- FaceIndexingService: a FileLifecycleHook that, on image upload, detects +
  embeds faces in a background task and stores them. Dedup-aware (clones an
  identical blob's faces instead of re-running inference), reindexes on
  overwrite, and relies on the DB cascade for deletes. Completely inert when
  no model is ready.
- DI: registers the hook in the FileLifecycleService chain and exposes
  PeopleService in AppState — both gated on OXICLOUD_ENABLE_FACES, both
  using the default no-op analyzer until the operator wires a real ONNX
  model.

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

src/infrastructure/services/mod.rs

@@ -6,6 +6,7 @@ pub mod compression_service;
 pub mod dedup_service;
 pub mod encrypted_blob_backend;
 pub mod exif_service;
+pub mod face_indexing_service;
 pub mod file_content_cache;
 pub mod file_system_i18n_service;
 pub mod image_transcode_service;