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/Persistence/ArchiveMetadataRecord.swift

@@ -0,0 +1,37 @@
+import Foundation
+import SwiftData
+
+/// Codable representation of a single durable MAM checkpoint, keyed by the
+/// archive it belongs to (account or a specific MUC).
+struct MAMCheckpointEntry: Codable, Equatable, Sendable {
+    let key: MAMArchiveKey
+    let checkpoint: MAMArchiveCheckpoint
+}
+
+/// Singleton per-account metadata that used to live in the JSON snapshot:
+/// locally deleted message IDs, roster contact JIDs and MAM checkpoints.
+@Model
+final class ArchiveMetadata {
+    @Attribute(.unique) var accountJID: String
+    var locallyDeletedMessageIDs: [String]
+    var rosterContactJIDs: [String]
+    var lastSuccessfulMAMSync: Date?
+    var lastSuccessfulMAMCursor: String?
+    var mamCheckpoints: [MAMCheckpointEntry]
+
+    init(
+        accountJID: String,
+        locallyDeletedMessageIDs: [String] = [],
+        rosterContactJIDs: [String] = [],
+        lastSuccessfulMAMSync: Date? = nil,
+        lastSuccessfulMAMCursor: String? = nil,
+        mamCheckpoints: [MAMCheckpointEntry] = []
+    ) {
+        self.accountJID = accountJID
+        self.locallyDeletedMessageIDs = locallyDeletedMessageIDs
+        self.rosterContactJIDs = rosterContactJIDs
+        self.lastSuccessfulMAMSync = lastSuccessfulMAMSync
+        self.lastSuccessfulMAMCursor = lastSuccessfulMAMCursor
+        self.mamCheckpoints = mamCheckpoints
+    }
+}