Phase 3: Extract response handling into classes/local/response/manager.php

Create mod_questionnaire\local\response\manager to centralise all response
CRUD and query operations previously scattered across questionnaire.class.php.

- New classes/local/response/manager.php with 17 methods:
  - Loading: add_user_responses, add_response, add_response_from_formdata,
    build_response_from_appdata, get_responses, user_has_saved_response
  - Saving: response_insert, response_commit, delete_insert_response,
    commit_submission_response
  - Deletion: response_delete
  - Validation: response_check_format
  - Querying: get_survey_all_responses, response_select, get_structured_response,
    get_full_submission_for_export
  - From locallib (for new callers): get_user_responses, delete_response,
    delete_responses

- questionnaire.class.php: replace each method body with a delegation call to
  responsemanager(); add responsemanager() lazy initialiser; make update_grades()
  public so the manager can call it.

locallib.php global functions are left unchanged — their callers pass questionnaire
IDs rather than objects and will be updated in a later phase.

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

Author: mchurchward