Upgrade to Django 6 (#20) (#26)

## Summary

Closes #20.

- Bump Django pin from `>=5.2,<6` to `>=6.0,<7` (resolved **6.0.7**) and refresh `uv.lock`
- Update `TimeInfoBase.save` for Django 6 keyword-only `Model.save`
- Pass `required_score` directly to `ReCaptchaV3` (removes django-recaptcha deprecation warning)
- Fix `Payments.date` fixed default → callable `timezone.now` (+ migration `0012`)

Reviewed against [Django 6.0 release notes](https://docs.djangoproject.com/en/6.0/releases/6.0/). `DEFAULT_AUTO_FIELD` already `BigAutoField`. No other removed APIs in use.

**Compat notes**
- `django-phonenumber-field` 8.4.0 advertises Django 6.0 support
- `django-recaptcha` 4.1.0 has no upper Django bound; classifiers still list through 5.2 only — smoke-tested via suite

## Test plan

- [x] `uv run python manage.py check` — clean
- [x] `uv run python manage.py test` — 30 passed
- [ ] CI unit tests green on this PR
- [ ] Smoke board pages, membership form, auth/sign-in, recaptcha after merge/deploy

Reviewed-on: #26

schasite/forms.py

@@ -19,11 +19,7 @@ class CaptchaForm(forms.Form):
     captcha = ReCaptchaField(
         public_key=settings.RECAPTCHA_PUBLIC_KEY,
         private_key=settings.RECAPTCHA_PRIVATE_KEY,
-        widget=ReCaptchaV3(
-            attrs={
-                'required_score':0.85,
-            }
-        ),
+        widget=ReCaptchaV3(required_score=0.85),
     )