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/child_api.py

@@ -1,5 +1,5 @@
 from time import sleep
-from datetime import date as date_type, datetime, timezone
+from datetime import datetime, timezone
 
 from flask import Blueprint, request, jsonify
 from tinydb import Query
@@ -31,7 +31,7 @@ from models.tracking_event import TrackingEvent
 from utils.tracking_logger import log_tracking_event
 from collections import defaultdict
 from db.chore_schedules import get_schedule
-from db.task_extensions import get_extension
+from db.task_extensions import get_extension_for_child_task
 import logging
 
 child_api = Blueprint('child_api', __name__)
@@ -288,8 +288,7 @@ def list_child_tasks(id):
         if task_obj.type == 'chore':
             schedule = get_schedule(id, tid)
             ct_dict['schedule'] = schedule.to_dict() if schedule else None
-            today_str = date_type.today().isoformat()
-            ext = get_extension(id, tid, today_str)
+            ext = get_extension_for_child_task(id, tid)
             ct_dict['extension_date'] = ext.date if ext else None
             
             # Attach pending confirmation status for chores