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/services/options.py

@@ -0,0 +1,23 @@
+"""Helpers for interpreting a Field's ``options`` (choice definitions).
+
+``options`` is a JSON list, either of plain strings (``["a", "b"]``) or of
+``{"value": ..., "label": ...}`` dicts.
+"""
+
+
+def field_choices(field):
+    """Return ``[(value, label), ...]`` for a choice-type field."""
+    choices = []
+    for option in field.options or []:
+        if isinstance(option, dict):
+            value = option.get("value")
+            label = option.get("label", value)
+        else:
+            value = label = option
+        choices.append((value, label))
+    return choices
+
+
+def allowed_values(field):
+    """Return the set of accepted values for a choice-type field."""
+    return {value for value, _ in field_choices(field)}