Save hash of captured photos

[gmaclennan]

Saving a hash of captured photos to the observation record would serve as proof that a photo was taken at a particular time, because the observation record is saved to hypercore which is immutable and signed, so it cannot be changed.

For historical reasons the filename / id for photos is a random string, not a hash. We could add another property to the attachments record which is a hash, e.g.

{
  "id": "79be849f934590ec",
  //...
  "attachments": [
    {
      "hash": {
        "type": "sha256",
        "value": "a97c005544ed29266329f17d7f7df638d6ebe6fe9b3077c0cfcb3c841366a4d1"
      },
      "id": "475704826aff65305e26723396f3d526.jpg"
    }
  ]
}

This would provide backwards compatibility with existing data - older photos would simply have no hashed. We could add a migration that adds hashes, but it wouldn't serve much purpose because it does not guarantee anything about the original photo.