Add Face

Create a placeholder face on a video file. The new face has no detection data — pair it with a speaker via Match Faces & Speakers when the AI missed a person. Auto-allocates face_id from a monotonic counter persisted on the doc (never reuses ids freed by delete). Cached CSV/PDF reports are invalidated since the report enumerates faces.

Returns the auto-allocated {face_id, name} so the FE can append the new tab without a re-read.