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/app/Models/FormBuilder/FormSchema.php

@@ -7,6 +7,7 @@ namespace App\Models\FormBuilder;
 use App\Enums\FormBuilder\FormPurpose;
 use App\Enums\FormBuilder\FormSchemaSnapshotMode;
 use App\Enums\FormBuilder\FormSubmissionMode;
+use App\Models\CrowdType;
 use App\Models\Organisation;
 use App\Models\Scopes\OrganisationScope;
 use App\Models\User;
@@ -42,6 +43,7 @@ final class FormSchema extends Model
         'name',
         'slug',
         'purpose',
+        'default_crowd_type_id',
         'description',
         'is_published',
         'submission_mode',
@@ -88,6 +90,12 @@ final class FormSchema extends Model
         return $this->belongsTo(Organisation::class);
     }
 
+    /** @return BelongsTo<CrowdType, $this> */
+    public function defaultCrowdType(): BelongsTo
+    {
+        return $this->belongsTo(CrowdType::class, 'default_crowd_type_id');
+    }
+
     public function owner(): MorphTo
     {
         return $this->morphTo();