refactor(form-builder): drop custom purpose escape from schemas

Reduces the FormPurpose vocabulary from 22 variants + a `custom` escape
to the seven v1.0 purposes registered in the new PurposeRegistry.

- Purge migration deletes any form_schemas row whose `purpose` is not
  in the v1.0 set (cascades through form_fields, form_submissions,
  form_values, form_value_options, form_schema_sections,
  form_submission_section_statuses, form_submission_delegations,
  form_schema_webhooks, form_webhook_deliveries via existing FK).
- Drop migration removes the `custom_purpose_slug` column + its index.
- Both migrations declare their `down()` as a hard failure — we do not
  support reversing a purge (pre-launch, no production data).
- `FormPurpose` enum slims to the seven cases; the legacy helpers
  (defaultSubmissionMode / defaultSubjectType / allowsPublicAccess)
  now delegate to PurposeRegistry so callers keep working.
- FormSchema fillable / FormSchemaResource / StoreFormSchemaRequest /
  UpdateFormSchemaRequest / FormSchemaFactory drop every reference to
  `custom_purpose_slug` and the `custom` purpose.
- VerifyFormsDataIntegrity drops the custom-slug mismatch check and
  sources the subject-type allow-list from PurposeRegistry.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/database/migrations/2026_04_24_100000_purge_invalid_form_purposes.php

@@ -0,0 +1,95 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\DB;
+
+/**
+ * WS-2 — Purge form_schemas rows whose `purpose` is no longer in the
+ * v1.0 PurposeRegistry (7-slug vocabulary) OR whose `purpose = 'custom'`.
+ *
+ * Child tables cascade via their existing FK constraints (cascadeOnDelete)
+ * on form_schemas/form_submissions.
+ *
+ * `down()` is intentionally a hard failure — we do not support reversing
+ * a purge migration (pre-launch, no production data).
+ */
+return new class extends Migration
+{
+    /** @var list<string> */
+    private const VALID_PURPOSES = [
+        'event_registration',
+        'artist_advance',
+        'supplier_intake',
+        'post_event_evaluation',
+        'incident_report',
+        'signature_contract',
+        'user_profile',
+    ];
+
+    public function up(): void
+    {
+        $invalidPurposes = DB::table('form_schemas')
+            ->whereNotIn('purpose', self::VALID_PURPOSES)
+            ->pluck('purpose')
+            ->unique()
+            ->values()
+            ->all();
+
+        if ($invalidPurposes === []) {
+            echo "[purge_invalid_form_purposes] no schemas with invalid purposes — nothing to delete.\n";
+
+            return;
+        }
+
+        $schemaIds = DB::table('form_schemas')
+            ->whereNotIn('purpose', self::VALID_PURPOSES)
+            ->pluck('id')
+            ->all();
+
+        $submissionIds = DB::table('form_submissions')
+            ->whereIn('form_schema_id', $schemaIds)
+            ->pluck('id')
+            ->all();
+
+        $counts = [
+            'form_schemas' => count($schemaIds),
+            'form_submissions' => count($submissionIds),
+            'form_fields' => DB::table('form_fields')->whereIn('form_schema_id', $schemaIds)->count(),
+            'form_schema_sections' => DB::table('form_schema_sections')->whereIn('form_schema_id', $schemaIds)->count(),
+            'form_values' => $submissionIds === [] ? 0
+                : DB::table('form_values')->whereIn('form_submission_id', $submissionIds)->count(),
+            'form_value_options' => $submissionIds === [] ? 0
+                : DB::table('form_value_options')
+                    ->whereIn('form_value_id', DB::table('form_values')->whereIn('form_submission_id', $submissionIds)->select('id'))
+                    ->count(),
+            'form_submission_section_statuses' => $submissionIds === [] ? 0
+                : DB::table('form_submission_section_statuses')->whereIn('form_submission_id', $submissionIds)->count(),
+            'form_submission_delegations' => $submissionIds === [] ? 0
+                : DB::table('form_submission_delegations')->whereIn('form_submission_id', $submissionIds)->count(),
+            'form_schema_webhooks' => DB::table('form_schema_webhooks')->whereIn('form_schema_id', $schemaIds)->count(),
+            'form_webhook_deliveries' => $submissionIds === [] ? 0
+                : DB::table('form_webhook_deliveries')->whereIn('form_submission_id', $submissionIds)->count(),
+        ];
+
+        echo sprintf(
+            "[purge_invalid_form_purposes] purging schemas with invalid purposes: %s\n",
+            implode(', ', $invalidPurposes),
+        );
+        foreach ($counts as $table => $n) {
+            echo "  {$table}: {$n}\n";
+        }
+
+        // FK cascade does the rest — delete only the parent rows.
+        DB::table('form_schemas')->whereIn('id', $schemaIds)->delete();
+    }
+
+    public function down(): void
+    {
+        throw new \RuntimeException(
+            'purge_invalid_form_purposes cannot be reversed — this migration '
+            .'deletes rows permanently. Restore from backup or run migrate:fresh.'
+        );
+    }
+};

api/database/migrations/2026_04_24_100001_drop_custom_purpose_slug_from_form_schemas.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+/**
+ * WS-2 — Drop the `custom_purpose_slug` escape from `form_schemas`.
+ *
+ * v1.0 Purpose registry replaces the `custom` purpose + per-organisation
+ * slug construct with a fixed set of seven purposes. See
+ * ARCH-CONSOLIDATION-2026-04.md §3 besluit 4 and ARCH-FORM-BUILDER.md
+ * §17.3 (Purpose registry).
+ *
+ * `down()` is intentionally a hard failure — we do not support reversing
+ * this migration (pre-launch, no production data).
+ */
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('form_schemas', function (Blueprint $table): void {
+            $table->dropIndex('fs_custom_purpose_slug_idx');
+            $table->dropColumn('custom_purpose_slug');
+        });
+    }
+
+    public function down(): void
+    {
+        throw new \RuntimeException(
+            'drop_custom_purpose_slug_from_form_schemas cannot be reversed — '
+            .'the custom purpose escape is retired in v1.0. Restore from '
+            .'backup or run migrate:fresh.'
+        );
+    }
+};