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/ArchiveStore.swift

@@ -0,0 +1,134 @@
+import Foundation
+import SwiftData
+
+/// SwiftData-backed replacement for the legacy JSON `ChatArchive`. Owns the
+/// per-account `ModelContainer`/`ModelContext`, imports the old JSON snapshot
+/// once, and exposes load/save/erase.
+@MainActor
+final class ArchiveStore {
+    struct Loaded {
+        var conversations: [Conversation]
+        var messages: [ChatMessage]
+        var locallyDeletedMessageIDs: Set<String>
+        var rosterContactJIDs: Set<String>
+        var lastSuccessfulMAMSync: Date?
+        var lastSuccessfulMAMCursor: String?
+        var mamCheckpoints: [MAMArchiveKey: MAMArchiveCheckpoint]
+    }
+
+    let container: ModelContainer
+    let context: ModelContext
+    private let accountJID: String
+    private let storeURL: URL
+    private let legacyJSONURL: URL
+    private(set) var metadata: ArchiveMetadata?
+
+    init(accountJID: String) throws {
+        self.accountJID = accountJID
+        let fileManager = FileManager.default
+        let base = fileManager.urls(for: .applicationSupportDirectory, in: .userDomainMask).first
+            ?? fileManager.temporaryDirectory
+        let directory = base.appendingPathComponent("Luma/Accounts", isDirectory: true)
+        try? fileManager.createDirectory(at: directory, withIntermediateDirectories: true)
+        let hash = Self.stableHash(accountJID)
+        storeURL = directory.appendingPathComponent("\(hash).store")
+        legacyJSONURL = directory.appendingPathComponent("\(hash).json")
+
+        let schema = Schema([
+            Conversation.self,
+            ChatMessage.self,
+            ArchiveMetadata.self,
+        ])
+        let configuration = ModelConfiguration(schema: schema, url: storeURL)
+        container = try ModelContainer(for: schema, configurations: [configuration])
+        context = ModelContext(container)
+        context.autosaveEnabled = false
+
+        migrateLegacyJSONIfNeeded()
+    }
+
+    func load() -> Loaded {
+        let conversationSort = [SortDescriptor(\Conversation.lastActivity, order: .reverse)]
+        let conversations = (try? context.fetch(FetchDescriptor<Conversation>(sortBy: conversationSort))) ?? []
+        let messages = (try? context.fetch(FetchDescriptor<ChatMessage>())) ?? []
+
+        let metadata = fetchMetadata()
+        let mamCheckpoints = Dictionary(
+            metadata.mamCheckpoints.map { ($0.key, $0.checkpoint) },
+            uniquingKeysWith: { first, _ in first }
+        )
+
+        return Loaded(
+            conversations: conversations,
+            messages: messages,
+            locallyDeletedMessageIDs: Set(metadata.locallyDeletedMessageIDs),
+            rosterContactJIDs: Set(metadata.rosterContactJIDs),
+            lastSuccessfulMAMSync: metadata.lastSuccessfulMAMSync,
+            lastSuccessfulMAMCursor: metadata.lastSuccessfulMAMCursor,
+            mamCheckpoints: mamCheckpoints
+        )
+    }
+
+    /// Persists the passed metadata plus all pending model mutations.
+    func save(
+        locallyDeletedMessageIDs: Set<String>,
+        rosterContactJIDs: Set<String>,
+        lastSuccessfulMAMSync: Date?,
+        lastSuccessfulMAMCursor: String?,
+        mamCheckpoints: [MAMArchiveKey: MAMArchiveCheckpoint]
+    ) throws {
+        let metadata = fetchMetadata()
+        metadata.locallyDeletedMessageIDs = Array(locallyDeletedMessageIDs)
+        metadata.rosterContactJIDs = Array(rosterContactJIDs)
+        metadata.lastSuccessfulMAMSync = lastSuccessfulMAMSync
+        metadata.lastSuccessfulMAMCursor = ArchiveSyncCheckpoint.normalizedCursor(lastSuccessfulMAMCursor)
+        metadata.mamCheckpoints = mamCheckpoints.map {
+            MAMCheckpointEntry(key: $0.key, checkpoint: $0.value)
+        }
+        try context.save()
+    }
+
+    func erase() throws {
+        try context.delete(model: Conversation.self)
+        try context.delete(model: ChatMessage.self)
+        try context.delete(model: ArchiveMetadata.self)
+        try context.save()
+    }
+
+    private func fetchMetadata() -> ArchiveMetadata {
+        if let metadata { return metadata }
+        let existing = try? context.fetch(FetchDescriptor<ArchiveMetadata>()).first
+        let metadata = existing ?? ArchiveMetadata(accountJID: accountJID)
+        context.insert(metadata)
+        self.metadata = metadata
+        return metadata
+    }
+
+    private func migrateLegacyJSONIfNeeded() {
+        guard FileManager.default.fileExists(atPath: legacyJSONURL.path) else { return }
+        guard let imported = try? LegacyArchiveImporter.importIfNeeded(from: legacyJSONURL) else {
+            return
+        }
+        for conversation in imported.conversations {
+            context.insert(conversation)
+        }
+        for message in imported.messages {
+            context.insert(message)
+        }
+        let metadata = fetchMetadata()
+        metadata.locallyDeletedMessageIDs = imported.locallyDeletedMessageIDs
+        metadata.rosterContactJIDs = imported.rosterContactJIDs
+        metadata.lastSuccessfulMAMSync = imported.lastSuccessfulMAMSync
+        metadata.lastSuccessfulMAMCursor = imported.lastSuccessfulMAMCursor
+        metadata.mamCheckpoints = imported.mamCheckpoints
+        try? context.save()
+        try? FileManager.default.removeItem(at: legacyJSONURL)
+    }
+
+    private static func stableHash(_ value: String) -> String {
+        let hash = value.lowercased().utf8.reduce(UInt64(14_695_981_039_346_656_037)) { partial, byte in
+            (partial ^ UInt64(byte)) &* 1_099_511_628_211
+        }
+        return String(hash, radix: 16, uppercase: false)
+    }
+}