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

pyproject.toml

@@ -5,7 +5,7 @@ description = "Django site for SCHA Wheaton"
 readme = "README.md"
 requires-python = ">=3.12"
 dependencies = [
-    "django>=5.2,<6",
+    "django>=6.0,<7",
     "django-phonenumber-field>=8.1.0",
     "django-recaptcha>=4.1.0",
     "gunicorn>=23.0.0",