fix: return extension_date regardless of server UTC date

list_child_tasks used date_type.today() (server UTC) to look up the
TaskExtension record. When the client's local date differed from the
server's UTC date (e.g. user is UTC-5 and it's past midnight UTC),
the lookup returned None and extension_date was incorrectly null.

Fix:
- Add get_extension_for_child_task() to db/task_extensions.py that
  queries by child_id + task_id only, without a date filter, and
  returns the entry with the latest date when multiple exist.
- Update list_child_tasks to use the new function. The comment already
  states 'client does all time math', so returning the stored
  extension_date is correct — the frontend compares it to local date.
- In extend_chore_time, delete any existing extension for the
  child+task before inserting the new one to prevent stale records
  from accumulating across days.
- Add regression test that inserts an extension for yesterday's date
  and asserts extension_date is non-null in the list-tasks response.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/api/chore_schedule_api.py

@@ -160,11 +160,14 @@ def extend_chore_time(child_id, task_id):
     if not date or not isinstance(date, str):
         return jsonify({'error': 'date is required (ISO date string)', 'code': ErrorCodes.MISSING_FIELD}), 400
 
-    # 409 if already extended for this date
+    # 409 if already extended for this exact date
     existing = get_extension(child_id, task_id, date)
     if existing:
         return jsonify({'error': 'Chore already extended for this date', 'code': 'ALREADY_EXTENDED'}), 409
 
+    # Clear any prior extension for this child+task before inserting the new one
+    # so stale records from previous dates do not accumulate.
+    delete_extension_for_child_task(child_id, task_id)
     extension = TaskExtension(child_id=child_id, task_id=task_id, date=date)
     add_extension(extension)