feat(channel): add graph-rooted model decorator

Mark the Channel model with `@Model({ name: 'Channel', graph: true })`
so each channel instance's triples are stored in an isolated named graph
within the perspective's Oxigraph store.

This depends on the named graphs feature in @coasys/ad4m (the
feat/subject-class-named-graphs branch).

Author: HexaField

packages/api/src/channel/index.ts

@@ -21,7 +21,7 @@ const {
   SUBGROUP_ITEM,
 } = community;
 
-@Model({ name: 'Channel' })
+@Model({ name: 'Channel', graph: true })
 export class Channel extends Ad4mModel {
   @Flag({ through: ENTRY_TYPE, value: EntryType.Channel })
   type: string;