feat(timetable): EngagementPortalController + /p/artist/{token}/* routes

Three backend endpoints under public throttle:30,1:
  GET  /p/artist/{token}                       — engagement summary + sections
  GET  /p/artist/{token}/sections/{section}    — form schema + draft values
  POST /p/artist/{token}/sections/{section}    — section submit

Token resolution via ArtistResolver::fromPortalToken (Step 2). The
master Artist becomes the FormSubmission subject; engagement.event_id
populates form_submissions.event_id per WS-4 denormalisation. Token
mismatches map to 404 (InvalidPortalTokenException), soft-deleted
master artists to 410 Gone (ArtistDeletedException).

Section submit reuses the existing FormBindingApplicator pipeline
(RFC-WS-6 v1.3.1) by dispatching FormSubmissionSectionSubmitted —
no parallel apply path. Drafts are idempotent on
'artist_advance:{engagement_id}', so repeated POSTs find the same
submission. AdvanceSection (engagement-scoped) ↔ FormSchemaSection
bridge: case-sensitive name match against the org's artist_advance
schema; the default seeder names them in lockstep.

Frontend in Session 5 — backend complete here.

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

api/app/Http/Resources/Api/V1/Portal/EngagementPortalResource.php

@@ -0,0 +1,64 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Resources\Api\V1\Portal;
+
+use App\Models\AdvanceSection;
+use App\Models\Artist;
+use App\Models\ArtistEngagement;
+use App\Models\Event;
+use Illuminate\Http\Request;
+use Illuminate\Http\Resources\Json\JsonResource;
+
+/**
+ * Public payload returned by GET /p/artist/{token}.
+ *
+ * Carries just enough for the portal to render the section
+ * navigation: artist + event identification, plus the engagement's
+ * AdvanceSection rows with their submission_status and ordering.
+ *
+ * @property ArtistEngagement $resource
+ */
+final class EngagementPortalResource extends JsonResource
+{
+    /**
+     * @return array<string, mixed>
+     */
+    public function toArray(Request $request): array
+    {
+        $engagement = $this->resource;
+        $artist = $engagement->getRelation('artist');
+        $event = $engagement->getRelation('event');
+
+        /** @var \Illuminate\Database\Eloquent\Collection<int, AdvanceSection> $sections */
+        $sections = $engagement->getRelation('advanceSections');
+
+        return [
+            'engagement_id' => (string) $engagement->id,
+            'artist' => $artist instanceof Artist ? [
+                'id' => (string) $artist->id,
+                'name' => (string) $artist->name,
+            ] : null,
+            'event' => $event instanceof Event ? [
+                'id' => (string) $event->id,
+                'name' => (string) $event->name,
+            ] : null,
+            'advancing_completed_count' => (int) $engagement->advancing_completed_count,
+            'advancing_total_count' => (int) $engagement->advancing_total_count,
+            'sections' => $sections
+                ->sortBy('sort_order')
+                ->values()
+                ->map(static fn (AdvanceSection $section): array => [
+                    'id' => (string) $section->id,
+                    'name' => (string) $section->name,
+                    'type' => $section->getRawOriginal('type'),
+                    'sort_order' => (int) $section->sort_order,
+                    'is_open' => (bool) $section->is_open,
+                    'submission_status' => $section->getRawOriginal('submission_status'),
+                    'last_submitted_at' => $section->getRawOriginal('last_submitted_at'),
+                ])
+                ->all(),
+        ];
+    }
+}