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/GroupConversationTests.swift

@@ -3,7 +3,7 @@ import XCTest
 @testable import Luma
 
 final class GroupConversationTests: XCTestCase {
-    func testGroupConversationRoundTrips() throws {
+    func testGroupConversationProperties() {
         let conversation = Conversation(
             jid: "team@conference.example.org",
             displayName: "Команда",
@@ -16,33 +16,11 @@ final class GroupConversationTests: XCTestCase {
             invitedBy: "bob@example.org"
         )
 
-        let decoded = try JSONDecoder().decode(
-            Conversation.self,
-            from: JSONEncoder().encode(conversation)
-        )
-
-        XCTAssertTrue(decoded.isGroup)
-        XCTAssertEqual(decoded.groupNickname, "Alice")
-        XCTAssertEqual(decoded.occupantCount, 4)
-        XCTAssertEqual(decoded.invitedBy, "bob@example.org")
-    }
-
-    func testLegacyConversationDefaultsToDirect() 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])
-        for key in ["kind", "groupNickname", "isGroupJoined", "shouldAutojoin", "occupantCount", "invitedBy"] {
-            object.removeValue(forKey: key)
-        }
-
-        let decoded = try JSONDecoder().decode(
-            Conversation.self,
-            from: JSONSerialization.data(withJSONObject: object)
-        )
-
-        XCTAssertFalse(decoded.isGroup)
-        XCTAssertFalse(decoded.isGroupJoined)
-        XCTAssertEqual(decoded.occupantCount, 0)
+        XCTAssertTrue(conversation.isGroup)
+        XCTAssertEqual(conversation.groupNickname, "Alice")
+        XCTAssertEqual(conversation.occupantCount, 4)
+        XCTAssertEqual(conversation.invitedBy, "bob@example.org")
+        XCTAssertEqual(conversation.encryptionPreference, .disabled)
     }
 
     func testNewGroupInheritsGlobalEncryptionByDefault() {
@@ -52,5 +30,6 @@ final class GroupConversationTests: XCTestCase {
         )
 
         XCTAssertEqual(conversation.encryptionPreference, .inheritGlobal)
+        XCTAssertTrue(conversation.isGroup)
     }
 }