fix: recaptcha_service KeyError protection and consistent None returns

[dreamcreated]

Summary

Fixes #858

Two bugs fixed in src/users/services/recaptcha_service.py → create_assessment():

1. Missing KeyError protection on reCAPTCHA API response

Before: Direct dictionary access response_data["tokenProperties"] raises an unhandled KeyError if the reCAPTCHA API returns an unexpected response format (network errors, API changes, outages). This crashes the signup flow with a 500 error.

After: Wrapped in try/except KeyError with a structured error log and return None, so the signup flow degrades gracefully.

2. Inconsistent return values ({} vs implicit None)

Before:

return {}    # invalid token path
return       # wrong action path  (implicit None)

After:

return None  # all failure paths

Both values are falsy, so the caller in views.py (if not assessment:) behaved correctly either way — but {} is semantically misleading (callers might assume a non-None dict means partial success) and makes the code fragile for future maintainers.

3. Minor: flattened else branch

The unnecessary else: after an early return None was removed to reduce nesting and improve readability.

Testing

Existing tests in src/users/tests/test_bot_protection.py cover the invalid-token, wrong-action, and low-score paths via mocked requests.post. All these tests continue to pass since:

• not None == True (same as before for the caller check)
• The mock data includes tokenProperties, so the new try/except path is not triggered in existing tests

A new test case could be added to cover the KeyError path (API returns response without tokenProperties), but that is left as a follow-up.