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/models.py

@@ -12,13 +12,13 @@ class TimeInfoBase(models.Model):
     class Meta:
         abstract = True
 
-    def save(self, *args, **kwargs):
+    def save(self, **kwargs):
         if not kwargs.pop("skip_last_modified", False) and not hasattr(self, "skip_last_modified"):
             self.last_modified = timezone.now()
             if kwargs.get("update_fields") is not None:
                 kwargs["update_fields"] = list({*kwargs["update_fields"], "last_modified"})
 
-        super().save(*args, **kwargs)
+        super().save(**kwargs)
 
 # Create your models here.
 class UsefulLinks(TimeInfoBase):
@@ -148,7 +148,7 @@ class MembershipServices(TimeInfoBase):
 
 
 class Payments(TimeInfoBase):
-    date = models.DateField(default=timezone.now())
+    date = models.DateField(default=timezone.now)
     status = models.CharField(default="Completed", max_length=256)
     email = models.EmailField(blank=True, null=True)
     person = models.ForeignKey(