feat: activate JSON schema validation for all new survey responses

[Joe-Heffer-Shef]

Context

PR #577 added the JSON Schema validation infrastructure (survey/schema.py, Survey.response_schema, SurveyResponse.clean()) but intentionally did not wire it into the live submission path. The service layer still uses SurveyResponse.objects.create() which skips full_clean(), so validation is not yet enforced on new responses.

The plan is to run python manage.py validate_responses on production data first to confirm clean baseline, then activate per-submission validation in this follow-up.

Work required

1. Activate validation on submit: Update survey/services/survey.py to delegate to the model method (which calls full_clean()):

   # survey/services/survey.py
   def accept_response(self, survey: Survey, responseValues):
       survey.accept_response(responseValues)  # delegates to Survey.accept_response() which calls full_clean()

2. Cache the schema: Change Survey.response_schema from @property to @cached_property so the JSON Schema is built once per instance rather than on every validation call.

   from django.utils.functional import cached_property
   
   @cached_property
   def response_schema(self) -> dict:
       ...

3. Update validate_responses output: Use ', '.join(exc.messages) instead of exc.message in the management command to handle multi-error ValidationError correctly.

Prerequisites

• Run python manage.py validate_responses on production and confirm 0 errors
• If errors are found, investigate and fix the data before merging this change

References

• PR feat: validate survey response answers against JSON Schema #577 — feat: validate survey response answers against JSON Schema