Storyline: per-chapter DM notes

Each chapter page gets a collapsible "Chapter notes" card at the top — a
free-text log the DM fills during play (the story the party tells after
the Crooked Goose, names given to NPCs, threads for next time). Saved
per chapter slug with a "saved <timestamp>" marker, same shape as the
Enemy/NPC game-notes field.

New: ChapterNote entity (slug-keyed, unique) + repository findOrCreate(),
ChapterNoteType form, chapter_note table migration (ENGINE = InnoDB
pinned). The storyline chapter route now also accepts POST to save.

Chapters stay hand-written Twig; only the mutable notes live in the DB.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/Form/ChapterNoteType.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace App\Form;
+
+use App\Entity\ChapterNote;
+use Symfony\Component\Form\AbstractType;
+use Symfony\Component\Form\Extension\Core\Type\TextareaType;
+use Symfony\Component\Form\FormBuilderInterface;
+use Symfony\Component\OptionsResolver\OptionsResolver;
+
+/**
+ * The DM's running log for a storyline chapter.
+ */
+class ChapterNoteType extends AbstractType
+{
+    public function buildForm(FormBuilderInterface $builder, array $options): void
+    {
+        $builder->add('notes', TextareaType::class, [
+            'required' => false,
+            'label' => false,
+            'attr' => [
+                'rows' => 12,
+                'placeholder' => "What happened at the table: the story the party told about the cottage, names they gave NPCs, promises made, threads to pick up next time…",
+            ],
+        ]);
+    }
+
+    public function configureOptions(OptionsResolver $resolver): void
+    {
+        $resolver->setDefaults(['data_class' => ChapterNote::class]);
+    }
+}