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/admin.py

@@ -0,0 +1,50 @@
+from django.contrib import admin
+
+from .models import Answer, Field, Form, Submission
+
+
+class FieldInline(admin.TabularInline):
+    model = Field
+    extra = 0
+    fields = ["order", "key", "label", "field_type", "required", "is_active"]
+    ordering = ["order"]
+
+
+@admin.register(Form)
+class FormAdmin(admin.ModelAdmin):
+    list_display = ["title", "slug", "club", "is_active", "login_required", "opens_at", "closes_at"]
+    list_filter = ["club", "is_active", "login_required"]
+    search_fields = ["title", "slug"]
+    prepopulated_fields = {"slug": ["title"]}
+    inlines = [FieldInline]
+
+
+@admin.register(Field)
+class FieldAdmin(admin.ModelAdmin):
+    list_display = ["label", "key", "form", "field_type", "order", "required", "is_active"]
+    list_filter = ["field_type", "required", "is_active"]
+    search_fields = ["label", "key", "form__title"]
+    raw_id_fields = ["form"]
+
+
+class AnswerInline(admin.TabularInline):
+    model = Answer
+    extra = 0
+    raw_id_fields = ["field"]
+
+
+@admin.register(Submission)
+class SubmissionAdmin(admin.ModelAdmin):
+    list_display = ["form", "member", "submitted_at"]
+    list_filter = ["form"]
+    search_fields = ["form__title", "member__first_name", "member__last_name"]
+    raw_id_fields = ["form", "member"]
+    readonly_fields = ["submitted_at"]
+    inlines = [AnswerInline]
+
+
+@admin.register(Answer)
+class AnswerAdmin(admin.ModelAdmin):
+    list_display = ["submission", "field", "value"]
+    search_fields = ["field__label", "field__key"]
+    raw_id_fields = ["submission", "field"]