refactor: add TypeConverters for Threads JSON columns (A4)

Add Drift TypeConverters for the two structured JSON columns in the
Threads table (participantsJson → List<EmailAddress>, emailIdsJson →
List<String>). The DB layer now owns serialisation for these fields;
_threadRowToModel no longer calls jsonDecode by hand.

Also extract _parseAddresses() helper to deduplicate the two identical
local functions that decoded Emails address columns.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/data/db/database.dart

@@ -6,8 +6,40 @@ import 'package:drift/native.dart';
 import 'package:path/path.dart' as p;
 import 'package:path_provider/path_provider.dart';
 
+import 'package:sharedinbox/core/models/email.dart';
+
 part 'database.g.dart';
 
+// ── TypeConverters ────────────────────────────────────────────────────────────
+
+class EmailAddressListConverter
+    extends TypeConverter<List<EmailAddress>, String> {
+  const EmailAddressListConverter();
+
+  @override
+  List<EmailAddress> fromSql(String fromDb) {
+    final list = jsonDecode(fromDb) as List<dynamic>;
+    return list
+        .map((e) => EmailAddress.fromJson(e as Map<String, dynamic>))
+        .toList();
+  }
+
+  @override
+  String toSql(List<EmailAddress> value) =>
+      jsonEncode(value.map((e) => e.toJson()).toList());
+}
+
+class StringListConverter extends TypeConverter<List<String>, String> {
+  const StringListConverter();
+
+  @override
+  List<String> fromSql(String fromDb) =>
+      List<String>.from(jsonDecode(fromDb) as List);
+
+  @override
+  String toSql(List<String> value) => jsonEncode(value);
+}
+
 // ── Tables ────────────────────────────────────────────────────────────────────
 
 class Accounts extends Table {
@@ -123,11 +155,14 @@ class Threads extends Table {
   IntColumn get messageCount => integer().withDefault(const Constant(1))();
   BoolColumn get hasUnread => boolean().withDefault(const Constant(false))();
   BoolColumn get isFlagged => boolean().withDefault(const Constant(false))();
-  // JSON-encoded List<{name,email}>
-  TextColumn get participantsJson => text().withDefault(const Constant('[]'))();
+  TextColumn get participantsJson => text()
+      .withDefault(const Constant('[]'))
+      .map(const EmailAddressListConverter())();
   TextColumn get preview => text().nullable()();
   TextColumn get latestEmailId => text()();
-  TextColumn get emailIdsJson => text().withDefault(const Constant('[]'))();
+  TextColumn get emailIdsJson => text()
+      .withDefault(const Constant('[]'))
+      .map(const StringListConverter())();
 
   @override
   Set<Column> get primaryKey => {accountId, mailboxPath, id};
@@ -411,10 +446,18 @@ class AppDatabase extends _$AppDatabase {
                   preview: Value(latest.preview),
                   latestEmailId: latest.id,
                   emailIdsJson: Value(
-                    jsonEncode(threadEmails.map((e) => e.id).toList()),
+                    threadEmails.map((e) => e.id).toList(),
                   ),
                   participantsJson: Value(
-                    latest.fromJson,
+                    (jsonDecode(latest.fromJson) as List<dynamic>)
+                        .map(
+                          (e) => EmailAddress(
+                            name:
+                                (e as Map<String, dynamic>)['name'] as String?,
+                            email: e['email'] as String,
+                          ),
+                        )
+                        .toList(),
                   ), // Good enough for migration
                 ),
               );

lib/data/repositories/email_repository_impl.dart

@@ -92,18 +92,6 @@ class EmailRepositoryImpl implements EmailRepository {
   }
 
   model.EmailThread _threadRowToModel(ThreadRow row) {
-    List<model.EmailAddress> parseAddresses(String json) {
-      final list = jsonDecode(json) as List<dynamic>;
-      return list
-          .map(
-            (e) => model.EmailAddress(
-              name: (e as Map<String, dynamic>)['name'] as String?,
-              email: e['email'] as String,
-            ),
-          )
-          .toList();
-    }
-
     return model.EmailThread(
       threadId: row.id,
       accountId: row.accountId,
@@ -113,10 +101,10 @@ class EmailRepositoryImpl implements EmailRepository {
       messageCount: row.messageCount,
       hasUnread: row.hasUnread,
       isFlagged: row.isFlagged,
-      participants: parseAddresses(row.participantsJson),
+      participants: row.participantsJson,
       preview: row.preview,
       latestEmailId: row.latestEmailId,
-      emailIds: List<String>.from(jsonDecode(row.emailIdsJson) as List),
+      emailIds: row.emailIdsJson,
     );
   }
 
@@ -156,13 +144,11 @@ class EmailRepositoryImpl implements EmailRepository {
 
     // Collect unique participants across the whole thread.
     final seen = <String>{};
-    final participants = <Map<String, dynamic>>[];
+    final participants = <model.EmailAddress>[];
     for (final e in threadEmails) {
-      final from = jsonDecode(e.fromJson) as List<dynamic>;
-      for (final a in from.cast<Map<String, dynamic>>()) {
-        final email = a['email'] as String;
-        if (seen.add(email)) {
-          participants.add({'name': a['name'], 'email': email});
+      for (final a in _parseAddresses(e.fromJson)) {
+        if (seen.add(a.email)) {
+          participants.add(a);
         }
       }
     }
@@ -177,12 +163,10 @@ class EmailRepositoryImpl implements EmailRepository {
             messageCount: Value(threadEmails.length),
             hasUnread: Value(threadEmails.any((e) => !e.isSeen)),
             isFlagged: Value(threadEmails.any((e) => e.isFlagged)),
-            participantsJson: Value(jsonEncode(participants)),
+            participantsJson: Value(participants),
             preview: Value(latest.preview),
             latestEmailId: latest.id,
-            emailIdsJson: Value(
-              jsonEncode(threadEmails.map((e) => e.id).toList()),
-            ),
+            emailIdsJson: Value(threadEmails.map((e) => e.id).toList()),
           ),
         );
   }
@@ -2708,18 +2692,6 @@ class EmailRepositoryImpl implements EmailRepository {
   }
 
   model.Email _toModel(Email row) {
-    List<model.EmailAddress> parseAddresses(String json) {
-      final list = jsonDecode(json) as List<dynamic>;
-      return list
-          .map(
-            (e) => model.EmailAddress(
-              name: (e as Map<String, dynamic>)['name'] as String?,
-              email: e['email'] as String,
-            ),
-          )
-          .toList();
-    }
-
     return model.Email(
       id: row.id,
       accountId: row.accountId,
@@ -2728,9 +2700,9 @@ class EmailRepositoryImpl implements EmailRepository {
       subject: row.subject,
       sentAt: row.sentAt,
       receivedAt: row.receivedAt,
-      from: parseAddresses(row.fromJson),
-      to: parseAddresses(row.toAddresses),
-      cc: parseAddresses(row.ccJson),
+      from: _parseAddresses(row.fromJson),
+      to: _parseAddresses(row.toAddresses),
+      cc: _parseAddresses(row.ccJson),
       preview: row.preview,
       isSeen: row.isSeen,
       isFlagged: row.isFlagged,
@@ -2766,6 +2738,18 @@ class EmailRepositoryImpl implements EmailRepository {
     }
   }
 
+  List<model.EmailAddress> _parseAddresses(String json) {
+    final list = jsonDecode(json) as List<dynamic>;
+    return list
+        .map(
+          (e) => model.EmailAddress(
+            name: (e as Map<String, dynamic>)['name'] as String?,
+            email: e['email'] as String,
+          ),
+        )
+        .toList();
+  }
+
   List<model.EmailAttachment> _parseAttachments(String json) {
     final list = jsonDecode(json) as List<dynamic>;
     return list

test/unit/email_repository_impl_test.dart

@@ -332,7 +332,7 @@ void main() {
               messageCount: const Value(2),
               hasUnread: const Value(true),
               latestEmailId: 'acc-1:2',
-              emailIdsJson: const Value('["acc-1:1", "acc-1:2"]'),
+              emailIdsJson: const Value(['acc-1:1', 'acc-1:2']),
             ),
           );