Automatically set post properties to photo properties

The logic proposal here is if a photo property is sent, evaluate image metadata to set post properties like publish date and location unless these properties are explicitly sent by the client.

The issue does not name files, tests, or an entry point. Start by tracing how the Micropub endpoint handles a photo property and how image metadata is read, then identify where post properties are assigned. Done means metadata supplies publish date and location only when the client has not explicitly provided them.

Written by the indexing model from the issue text.