Store Image and Embedding Without Subject

**Is your feature request related to a problem? Please describe.**
I'm looking to store embeddings without a specific subject.

**Describe the solution you'd like**
An API Endpoint to add an image and embedding without a subject. From what I see in the docs and code, the subject is currently required even though the column is nullable.

**Describe alternatives you've considered**
Manually saving the image and embeddings.

**Additional context**
The goal is to be able to search a given image against previously saved unknown faces.

Start by tracing the image and embedding API in the documentation and code, then inspect the storage model where the subject column is nullable. Define the endpoint behavior for saving records without a subject and verify that an image can be searched against those records without manual database writes.

Written by the indexing model from the issue text.