feat(faces): real ONNX face analyzer (SCRFD + ArcFace), opt-in

Implements the last Phase 2 piece: a working face detector/embedder behind
the new `faces-onnx` cargo feature (mirrors how `plugins` gates wasmtime).
Inert by default — the default build is unchanged and ships the no-op
analyzer.

Pipeline (InsightFace/immich pattern): SCRFD detection with 5-point
landmarks → least-squares similarity alignment to the canonical 112×112
template → ArcFace embedding → L2-normalized 512-d vector.

- face_geometry.rs (always compiled, unit-tested): SCRFD anchor/distance
  decode, NMS, the closed-form (complex-number) similarity transform,
  bilinear affine warp, NCHW normalization, L2-norm, Laplacian sharpness.
  11 unit tests cover the error-prone math with no model needed.
- onnx_face_analyzer.rs (feature `faces-onnx`): wires the geometry to ONNX
  Runtime via `ort` (load-dynamic, so libonnxruntime is dlopen'd at runtime
  and the crate builds without it). Inference runs on spawn_blocking; each
  session is serialized behind a Mutex. Loads via `ort::init_from` (fallible)
  not ORT's lazy loader, which would panic under `panic = "abort"`.
- config: FacesConfig + OXICLOUD_FACES_{ORT_DYLIB,DETECTOR_MODEL,
  EMBEDDER_MODEL,DET_SIZE,DET_THRESHOLD,NMS_THRESHOLD,INTRA_THREADS}.
- di: build_face_analyzer() loads the real analyzer when the feature is
  compiled in and runtime+models are configured; any missing piece or load
  failure degrades to the no-op analyzer (logged) so startup never fails.
- ort/ndarray added as optional deps; example.env documents the setup.

Models and the ONNX Runtime dylib are operator-provided at runtime and are
never committed. Cannot be exercised in CI (no models/dylib); the geometry
is unit-tested and the ONNX seam is isolated.

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_geometry;
 pub mod face_indexing_service;
 pub mod file_content_cache;
 pub mod file_system_i18n_service;
@@ -20,6 +21,8 @@ pub mod mock_email_sender;
 pub mod nextcloud_chunked_upload_service;
 pub mod noop_face_analyzer;
 pub mod oidc_service;
+#[cfg(feature = "faces-onnx")]
+pub mod onnx_face_analyzer;
 pub mod password_hasher;
 pub mod path_resolver_service;
 pub mod path_service;