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.

Scripts/verify.sh

@@ -253,8 +253,8 @@ grep -q 'decodeSliceSize = 8' Sources/Shared/Models/ArchiveMessageBatchPolicy.sw
   echo "MAM decoding must stay batched in small per-slice groups"
   exit 1
 }
-grep -q 'lastSuccessfulMAMCursor' Sources/Shared/Persistence/ChatArchive.swift || {
-  echo "The durable archive snapshot must persist the XEP-0313 UID cursor"
+grep -q 'lastSuccessfulMAMCursor' Sources/Shared/Persistence/ArchiveStore.swift || {
+  echo "The durable archive store must persist the XEP-0313 UID cursor"
   exit 1
 }
 grep -q 'ArchiveSyncCursorPolicy.requestPosition' Sources/Shared/XMPP/XMPPService.swift || {