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>

tests/Entity/ChapterNoteTest.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace App\Tests\Entity;
+
+use App\Entity\ChapterNote;
+use PHPUnit\Framework\TestCase;
+
+class ChapterNoteTest extends TestCase
+{
+    public function testStartsEmptyForItsSlug(): void
+    {
+        $note = new ChapterNote('prologue-the-long-road');
+
+        self::assertSame('prologue-the-long-road', $note->getSlug());
+        self::assertNull($note->getId());
+        self::assertNull($note->getNotes());
+        self::assertNull($note->getUpdatedAt());
+    }
+
+    public function testTouchStampsUpdatedAt(): void
+    {
+        $note = new ChapterNote('overview');
+        $note->setNotes("The party told Maud the cottage was haunted.");
+        $note->touch();
+
+        self::assertNotNull($note->getUpdatedAt());
+        self::assertSame("The party told Maud the cottage was haunted.", $note->getNotes());
+    }
+
+    public function testNotesCanBeClearedBackToNull(): void
+    {
+        $note = (new ChapterNote('overview'))->setNotes('something');
+
+        $note->setNotes(null);
+
+        self::assertNull($note->getNotes());
+    }
+}