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/tests/test_child_api.py

@@ -439,6 +439,29 @@ def test_list_child_tasks_returns_extension_date_when_set(client):
     assert tasks[TASK_GOOD_ID]['extension_date'] == today
 
 
+def test_list_child_tasks_returns_extension_date_for_yesterday(client):
+    """Regression: extension inserted for yesterday's date must still be returned.
+
+    Previously, list-tasks used date_type.today() on the server to look up
+    extensions, causing a timezone mismatch when the client's local date
+    differed from the server's UTC date.  The fix removes the date filter so
+    any stored extension is returned regardless of which date it was created
+    for.
+    """
+    _setup_sched_child_and_tasks(task_db, child_db)
+    from datetime import date as date_type_local, timedelta
+    yesterday = (date_type_local.today() - timedelta(days=1)).isoformat()
+    task_extensions_db.insert({
+        'id': 'ext-yesterday',
+        'child_id': CHILD_SCHED_ID,
+        'task_id': TASK_GOOD_ID,
+        'date': yesterday,
+    })
+    resp = client.get(f'/child/{CHILD_SCHED_ID}/list-tasks')
+    tasks = {t['id']: t for t in resp.get_json()['tasks']}
+    assert tasks[TASK_GOOD_ID]['extension_date'] == yesterday
+
+
 def test_list_child_tasks_extension_date_null_when_not_set(client):
     """Good chore with no extension returns extension_date=null."""
     _setup_sched_child_and_tasks(task_db, child_db)