feat : add compute_event_embedding to FaceEmbeddingService

[maya-ots]

summary

added compute_event_embedding() to FaceEmbeddingService to handle event photo processing. Unlike compute_average_embedding() which is designed for enrollment (one person, multiple photos), this method detects all faces in one or more photos and returns one embedding per face per photo.

changes

• added compute_event_embedding() to FaceEmbeddingService (takes a list of image payloads, returns a dict of filename → list of embeddings)
• each embedding is 512-dimensional, one per detected face in the photo.

testing

• tested on a group photo with 5 people, 5 faces detected, each returned a 512-dim embedding
• ruff and mypy checks pass with no issues