feat(rotation): per-device image-selection preferences

Adds two settings exposed in the PWA frame-settings sheet:

- rotationMode (enum: random | least_recently_shown | oldest_upload |
  newest_upload). Default oldest_upload preserves the legacy
  hard-coded sort, so existing devices behave identically until the
  user changes it.
- prioritizeNeverShown (bool). When set, the candidate set is narrowed
  to never-shown images first (if any exist) before the mode runs —
  useful for "burn through new uploads before re-shuffling the catalog."

RotationService pipeline:
  1. Pull approved/ready pool.
  2. Drop the last `uniquenessWindow` served (existing).
  3. If prioritizeNeverShown AND any candidates have never been served,
     narrow to those.
  4. Apply the selection mode.

Backend: enum, entity columns + accessors, migration, serializer,
PATCH validator. Frontend: types, stores, settings sheet section
(dropdown + checkbox), test fixtures, save-flow test.

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

migrations/Version20260507230003.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260507230003 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add device.rotation_mode + device.prioritize_never_shown — image-selection preferences exposed in the PWA settings sheet';
+    }
+
+    public function up(Schema $schema): void
+    {
+        // Default 'oldest_upload' preserves the legacy hard-coded behavior so
+        // existing devices keep their current rotation order with no surprise.
+        $this->addSql("ALTER TABLE device ADD rotation_mode VARCHAR(32) NOT NULL DEFAULT 'oldest_upload'");
+        $this->addSql('ALTER TABLE device ADD prioritize_never_shown BOOLEAN NOT NULL DEFAULT FALSE');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE device DROP COLUMN rotation_mode');
+        $this->addSql('ALTER TABLE device DROP COLUMN prioritize_never_shown');
+    }
+}