feat(todo): comments on a task (ADR 0033, D36)

Plain text with links, written, edited and deleted in the task's sheet,
counted on its row. They are sealed in `task_comment`, the task they belong
to with them, and go with their task.

Whats-New: Comments: write on a task, edit or delete what you wrote, and open its links.
Whats-New-De: Kommentare: Schreibe zu einer Aufgabe, bearbeite oder lösche es und öffne Links darin.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/todo/halkyon_todo_server/test/integration/project/person_deletion_test.dart

@@ -53,7 +53,7 @@ void main() {
 
       test(
         'deleting a person deletes their space with every key, project, '
-        "section, task, label, filter and settings in it, and nobody else's",
+        "section, task, label, filter, comment and settings in it, and nobody else's",
         () async {
           for (final person in [alice, bob]) {
             await _projectOf(session, person);
@@ -107,6 +107,7 @@ const _tables = [
   'task',
   'label',
   'saved_filter',
+  'task_comment',
   'todo_settings',
 ];
 
@@ -152,4 +153,9 @@ Future<void> _projectOf(Session session, UuidValue person) =>
         TodoSettings(keyId: 'todo1.0.key', payload: sealed, nonce: sealed),
         transaction: transaction,
       );
+      await TaskComment.db.insertRow(
+        session,
+        TaskComment(keyId: 'todo1.0.key', payload: sealed, nonce: sealed),
+        transaction: transaction,
+      );
     });