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>

templates/storyline/chapter.html.twig

@@ -22,6 +22,24 @@
         </nav>
 
         <div class="col-lg-9 order-lg-1">
+            <details class="card shadow-sm mb-4 storyline-notes" open>
+                <summary class="card-header d-flex justify-content-between align-items-center">
+                    <span>📝 Chapter notes</span>
+                    {% if note.updatedAt %}
+                        <span class="text-body-secondary small fw-normal">saved {{ note.updatedAt|date('Y-m-d H:i') }}</span>
+                    {% endif %}
+                </summary>
+                <div class="card-body">
+                    <p class="text-body-secondary small mb-2">
+                        Your running log for this chapter — what actually happened at the table.
+                    </p>
+                    {{ form_start(form) }}
+                    {{ form_row(form.notes) }}
+                    <button class="btn btn-primary mt-2">Save notes</button>
+                    {{ form_end(form) }}
+                </div>
+            </details>
+
             <article class="storyline-doc">
                 <header class="storyline-doc-head">
                     <div class="storyline-eyebrow">Storyline &middot; {{ chapter.eyebrow }}</div>