feat(formbuilder): fix tenancy bugs, add admin + services

Fix multi-tenancy/integrity bugs in the models: Form.slug and Field.key
were globally unique (unique=True), so two clubs couldn't reuse a form
slug and two forms couldn't reuse a field key — make slug unique per club
(constraint already present) and key unique per form. Add a
(submission, field) uniqueness constraint on Answer.

Register all four models in the admin (Field inline on Form, Answer inline
on Submission) and add formbuilder to the admin registration smoke test.

Add a service layer:
- submit_form(form, member, data): enforces is_active / login_required /
  open window / max_submissions, validates required + choice fields, and
  writes a Submission with Answers atomically (FormSubmissionError carries
  per-field errors).
- build_form(form): a live django.forms.Form built from a Form's active
  fields, mapping each FieldType to the matching form field.
- form_report(form): a tabular overview of every submission's answers plus
  per-value tallies for choice-type fields.

Full suite at 100% coverage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

formbuilder/migrations/0003_fix_tenant_uniqueness.py

@@ -0,0 +1,31 @@
+# Generated by Django 6.0.6 on 2026-07-12 19:05
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('formbuilder', '0002_alter_answer_field_alter_submission_member'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='field',
+            name='key',
+            field=models.SlugField(verbose_name='key'),
+        ),
+        migrations.AlterField(
+            model_name='form',
+            name='slug',
+            field=models.SlugField(verbose_name='slug'),
+        ),
+        migrations.AddConstraint(
+            model_name='answer',
+            constraint=models.UniqueConstraint(fields=('submission', 'field'), name='unique_answer_per_field_per_submission'),
+        ),
+        migrations.AddConstraint(
+            model_name='field',
+            constraint=models.UniqueConstraint(fields=('form', 'key'), name='unique_field_key_per_form'),
+        ),
+    ]