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.

Tests/EncryptionPreferenceTests.swift

@@ -10,19 +10,6 @@ final class EncryptionPreferenceTests: XCTestCase {
         XCTAssertFalse(EncryptionPreference.disabled.resolved(globalEnabled: true))
     }
 
-    func testConversationWithoutEncryptionFieldStillDecodes() throws {
-        let conversation = Conversation(jid: "bob@example.org", displayName: "Bob")
-        let encoded = try JSONEncoder().encode(conversation)
-        var object = try XCTUnwrap(JSONSerialization.jsonObject(with: encoded) as? [String: Any])
-        object.removeValue(forKey: "encryptionPreference")
-        let legacyData = try JSONSerialization.data(withJSONObject: object)
-
-        let decoded = try JSONDecoder().decode(Conversation.self, from: legacyData)
-
-        XCTAssertEqual(decoded.encryptionPreference, .inheritGlobal)
-        XCTAssertEqual(decoded.jid, conversation.jid)
-    }
-
     func testGlobalSettingIsStoredPerAccount() throws {
         let suite = "LumaTests.\(UUID().uuidString)"
         let defaults = try XCTUnwrap(UserDefaults(suiteName: suite))