feat: add logging for validation failures and misconfiguration

[tdruez]

New documentation from README:

## Logging

Django Altcha uses the standard Python `logging` module under the logger name
`django_altcha`. No logs are emitted under normal operation; logging fires only
on validation failures and misconfiguration.

### What gets logged

- **WARNING** on invalid or missing CAPTCHA tokens submitted to a form.
- **WARNING** on replay attempts (a challenge reused after it has already been validated).
- **ERROR** when `ALTCHA_HMAC_KEY` is not configured.
- **Exception with traceback** when verification or payload decoding raises
  unexpectedly.

Payloads, challenge values, and the HMAC key are never included in log
messages.

### Enabling logs

Add the `django_altcha` logger to your project's `LOGGING` setting:

LOGGING = {
    "version": 1,
    "disable_existing_loggers": False,
    "handlers": {
        "console": {
            "class": "logging.StreamHandler",
        },
    },
    "loggers": {
        "django_altcha": {
            "handlers": ["console"],
            "level": "WARNING",
        },
    },
}

Set `"level": "ERROR"` to see only misconfiguration and unexpected failures,
or `"level": "DEBUG"` to see additional diagnostic messages during development.