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/migrations/0012_alter_payments_date.py

@@ -0,0 +1,19 @@
+# Generated by Django 6.0.7 on 2026-07-14 12:13
+
+import django.utils.timezone
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('schasite', '0011_alter_payments_date_communitymember_and_more'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='payments',
+            name='date',
+            field=models.DateField(default=django.utils.timezone.now),
+        ),
+    ]