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>

migrations/Version20260908164955.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20260908164955 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return "Add chapter_note table for the DM's per-chapter storyline notes";
+    }
+
+    public function up(Schema $schema): void
+    {
+        // ENGINE = InnoDB is pinned explicitly: the local WAMP MariaDB defaults
+        // to MyISAM, which would silently drop the unique index behaviour.
+        $this->addSql('CREATE TABLE chapter_note (id INT AUTO_INCREMENT NOT NULL, slug VARCHAR(120) NOT NULL, notes LONGTEXT DEFAULT NULL, updated_at DATETIME DEFAULT NULL, UNIQUE INDEX uniq_chapter_note_slug (slug), PRIMARY KEY (id)) DEFAULT CHARACTER SET utf8mb4 ENGINE = InnoDB');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('DROP TABLE chapter_note');
+    }
+}