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/Services/ChatMediaImageCache.swift

@@ -24,7 +24,7 @@ enum ChatMediaImageCache {
         for message: ChatMessage,
         data: Data
     ) -> ChatMediaPlatformImage? {
-        let cacheKey = "\(message.conversationID)\u{1F}\(message.id)" as NSString
+        let cacheKey = "\(message.conversationID)\u{1F}\(message.clientID)" as NSString
         if let cached = cache.object(forKey: cacheKey) {
             return cached
         }

Sources/Shared/Services/PhoneWatchBridge.swift

@@ -82,7 +82,7 @@ final class PhoneWatchBridge: NSObject, WCSessionDelegate {
         messages: [ChatMessage]
     ) -> Data? {
         let visibleConversations = Array(conversations.prefix(20))
-        let visibleConversationIDs = Set(visibleConversations.map(\.id))
+        let visibleConversationIDs = Set(visibleConversations.map(\.jid))
         var recentMessagesByConversation: [String: [ChatMessage]] = [:]
 
         // Keep a bounded, chronologically sorted window. MAM can append an old
@@ -103,10 +103,10 @@ final class PhoneWatchBridge: NSObject, WCSessionDelegate {
                 jid: conversation.jid,
                 name: conversation.displayName,
                 unread: conversation.unreadCount,
-                messages: (recentMessagesByConversation[conversation.id] ?? [])
+                messages: (recentMessagesByConversation[conversation.jid] ?? [])
                     .map {
                         PhoneWatchSnapshot.Message(
-                            id: $0.id,
+                            id: $0.clientID,
                             body: $0.previewText,
                             timestamp: $0.timestamp,
                             outgoing: $0.direction == .outgoing,
@@ -128,7 +128,7 @@ final class PhoneWatchBridge: NSObject, WCSessionDelegate {
             let middle = lowerBound + (upperBound - lowerBound) / 2
             let existing = sortedMessages[middle]
             let existingComesFirst = existing.timestamp < message.timestamp
-                || (existing.timestamp == message.timestamp && existing.id < message.id)
+                || (existing.timestamp == message.timestamp && existing.clientID < message.clientID)
             if existingComesFirst {
                 lowerBound = middle + 1
             } else {