Etcher.Storage adapter that writes annotation events from the
MediaBrowser's Fresco viewer into phoenix_kit_annotations.
Etcher's generic API is keyed by target_type + target_uuid so the
library can annotate any kind of resource. In PhoenixKit the only
target is a media File, so this adapter requires target_type == "file" and maps target_uuid to file_uuid.
Comment threads
An annotation's discussion thread is not created at draw time —
it's instantiated lazily when the user posts the first comment on the
annotation. The comments are anchored to the file
(resource_type = "file", resource_uuid = file_uuid) with
metadata.annotation_uuid carrying the back-reference, so they
appear in the file's main thread alongside non-annotated discussion.
No comment_uuid column on annotations is needed.