migrate persistence from JSON to SwiftData

Models:
- Convert Conversation and ChatMessage to SwiftData @Model classes with
  a
  one-to-many relationship and cascade delete.
- Rename ChatMessage.id to clientID and drop Conversation.id in favor of
  jid
  (the string id would clash with PersistentIdentifier).
- Keep MessageReaction as a Codable value stored in an array attribute.

Persistence:
- Add ArchiveStore, a per-account ModelContainer/ModelContext that
  replaces
  the JSON ChatArchive with load/save/erase.
- Add ArchiveMetadataRecord for the durable MAM checkpoints, cursor,
  locally
  deleted message IDs and roster contact JIDs.
- Add LegacyArchiveImporter to import the old JSON snapshot once and
  delete
  the file afterwards, preserving existing history.

AppModel:
- Replace ChatArchive with ArchiveStore and persist via context.save();
  insert new models and delete the replaced object on upsert merges so
  SwiftData never keeps an orphaned duplicate.

Tests:
- Replace ChatArchiveTests with ArchiveStoreTests (store round-trip and
  legacy import) and drop the now-obsolete Codable round-trip
  assertions.

Sources/Shared/UI/Components/VideoNotePreview.swift

@@ -82,7 +82,7 @@ struct VideoNotePreview: View {
         }
         .onReceive(NotificationCenter.default.publisher(for: .lumaExclusiveMediaPlayback)) { notification in
             guard let activeID = notification.object as? String,
-                  activeID != message.id else { return }
+                  activeID != message.clientID else { return }
             player?.pause()
             isPlaying = false
         }
@@ -146,7 +146,7 @@ struct VideoNotePreview: View {
             model.audioPlayback.stop()
             NotificationCenter.default.post(
                 name: .lumaExclusiveMediaPlayback,
-                object: message.id
+                object: message.clientID
             )
             player.play()
         }