feat(form-builder): per-schema default_crowd_type_id replaces silent oldest() heuristic (WS-6)

Session 2's PersonProvisioner picked CrowdType::oldest() for the org —
silently wrong for multi-crowd_type orgs (Volunteer + Crew + Press are
three distinct crowd_types in one org). Schemas now declare their
target crowd_type explicitly via form_schemas.default_crowd_type_id.
RequiresDefaultCrowdType publish guard prevents misconfigured
event_registration schemas from publishing.

PersonProvisioner: oldest() fallback removed entirely. Misconfiguration
throws no_default_crowd_type at runtime; publish guard prevents it at
config time.

Migration uses a plain ulid() column without DB-level FK because
SQLite's table-rebuild on ALTER ADD FOREIGN KEY cascade-deletes
form_fields rows (form_fields.form_schema_id has cascadeOnDelete on
form_schemas). Application-level integrity via FormSchema::defaultCrowdType()
belongsTo + the publish guard + the runtime failsafe — three load-bearing
checks, none of which require the DB-level constraint.

Three pre-existing migration backfill tests bumped step counts +1 to
account for the new migration sitting between WS-5c and WS-5d:
FormFieldBindingMigrationTest (16→17, 14→15), FormFieldConfigBackfillAndDropTest
(11→12), FormFieldValidationRuleBackfillTest (14→15),
ConditionalLogicBackfillTest (5→6).

Six event_registration test fixtures updated to set default_crowd_type_id
to satisfy the new publish guard.

FormBuilderDevSeeder.resolveDefaultCrowdTypeId() — VOLUNTEER → first-active
→ create-as-needed fallback chain; documented contract for future seeders.

SCHEMA.md updated to v2.7.
Refs: RFC-WS-6.md v1.1 §3 Q8 addendum (Task 4 of this session)

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

api/database/migrations/2026_04_26_120000_add_default_crowd_type_id_to_form_schemas.php

@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+/**
+ * RFC-WS-6 v1.1 §3 Q8 addendum — per-schema default CrowdType for
+ * Person provisioning. Replaces the silent oldest() heuristic from
+ * WS-6 session 2 with explicit configuration; RequiresDefaultCrowdType
+ * publish guard ensures event_registration schemas declare it.
+ *
+ * No backfill: pre-launch the table is empty. Dev seeders populate
+ * the column when reseeding (FormBuilderDevSeeder).
+ */
+return new class extends Migration
+{
+    public function up(): void
+    {
+        // Plain nullable ULID column. NO database-level foreign key —
+        // SQLite's rebuild-on-FK-add cascade-deletes form_fields rows
+        // (form_fields.form_schema_id has cascadeOnDelete on
+        // form_schemas), which corrupts running migration tests.
+        // Application-level integrity: FormSchema::defaultCrowdType()
+        // belongsTo loads from CrowdType; CrowdTypeObserver could add
+        // a soft-handle on delete if needed in production. The new
+        // RequiresDefaultCrowdType publish guard plus the runtime
+        // failsafe in PersonProvisioner are the load-bearing checks.
+        Schema::table('form_schemas', function (Blueprint $table): void {
+            $table->ulid('default_crowd_type_id')
+                ->nullable()
+                ->after('purpose');
+
+            $table->index(['organisation_id', 'default_crowd_type_id'], 'fs_org_default_crowd_type_idx');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('form_schemas', function (Blueprint $table): void {
+            $table->dropIndex('fs_org_default_crowd_type_idx');
+            $table->dropColumn('default_crowd_type_id');
+        });
+    }
+};