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>

assets/styles/app.css

@@ -429,6 +429,23 @@ table.dataTable {
 }
 .storyline-pager-title { font-weight: 600; }
 
+/* Chapter notes: collapsible card above the chapter body */
+.storyline-notes > summary {
+    cursor: pointer;
+    list-style: none;
+    font-weight: 600;
+}
+.storyline-notes > summary::-webkit-details-marker { display: none; }
+.storyline-notes > summary::before {
+    content: "\25B8";              /* right-pointing triangle */
+    display: inline-block;
+    margin-right: 0.5rem;
+    color: var(--bs-secondary-color);
+    transition: transform 0.15s ease;
+}
+.storyline-notes[open] > summary::before { transform: rotate(90deg); }
+.storyline-notes textarea { font-size: 0.95rem; line-height: 1.6; }
+
 /* Chapter list: kicker line above each title */
 .storyline-chapter-kicker {
     display: block;