Finalize avatar upload

Finalize a previously uploaded avatar image and apply it to the authenticated user account.

This endpoint is step 3 of the avatar workflow and must be called after uploading the image binary.

Finalization behavior:

• Validates the provided image identifier.
• Confirms upload completion.
• Performs content checks before applying the image.
• Rejects disallowed content without updating the avatar.

Typical sequence:

1. POST /user/account/avatar
2. Upload image to returned URL.
3. PATCH /user/account/avatar, which is this endpoint.