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/FormBuilder/Purposes/Guards/EventRegistrationGuards.php

@@ -12,6 +12,7 @@ use App\FormBuilder\Publishing\IdentityKeyBindingsOnlyInFirstSection;
 use App\FormBuilder\Publishing\MaxOneIdentityKeyPerTargetEntity;
 use App\FormBuilder\Publishing\NoAmbiguousTrustLevels;
 use App\FormBuilder\Publishing\RequiresFieldType;
+use App\FormBuilder\Publishing\RequiresDefaultCrowdType;
 use App\FormBuilder\Publishing\RequiresIdentityKeyBinding;
 use App\FormBuilder\Publishing\SchemaHasLinkedEvent;
 use App\FormBuilder\Publishing\TagCategoriesConfiguredOnAllPickers;
@@ -27,6 +28,7 @@ final readonly class EventRegistrationGuards implements PurposeGuardProvider
     {
         return [
             new RequiresIdentityKeyBinding('person', 'email'),
+            new RequiresDefaultCrowdType(),
             new MaxOneIdentityKeyPerTargetEntity(),
             new RequiresFieldType(FormFieldType::EMAIL, 1),
             new ConditionalRequirement(