Storyline: add the Prologue chapter, extend the layout for sessions

Chapters now carry an eyebrow ("Overview", "Prologue", ...) and a short
tag for the list/sidebar badge, instead of a bare running number, so
non-numbered entries read correctly.

New chapter: "Prologue - The Long Road to Brackenford", the session
outline formatted as a runnable document -- an at-a-glance metadata band
(levels, duration, locations, hook), the in-world courier notice as a
posted handout, numbered scenes, GM callouts for the "ask the players"
prompts, label/body rows for the skill-check clues and travel-day
prompts, a three-stage encounter block, dramatic beat lines, and a
before/after panel for the legend the party's own retelling becomes.
Cross-links the Hearthburrow to Enemies and the closing line to the
overview's "Five and the Five".

Tests: render every chapter partial headless (no HTTP/DB) so a broken
variable or path() fails loudly, check each registered chapter has its
template file, and assert the /storyline routes sit behind the login gate.

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

templates/storyline/index.html.twig

@@ -8,8 +8,9 @@
             <div class="text-uppercase small text-body-secondary fw-semibold" style="letter-spacing:.08em">Storyline</div>
             <h1 class="display-6 mb-2">Brackenford</h1>
             <p class="text-body-secondary mb-0" style="max-width:60ch">
-                The campaign bible, split into chapters. Start with the overview for the whole
-                arc; later chapters will cover sessions, locations and the people of the village.
+                The campaign bible, split into chapters. The overview lays out the whole arc;
+                the prologue is the first session. More chapters — sessions, locations and the
+                people of the village — will follow.
             </p>
         </header>
 
@@ -17,8 +18,9 @@
             {% for slug, chapter in chapters %}
                 <li>
                     <a href="{{ path('app_storyline_chapter', {slug: slug}) }}" class="storyline-chapter-card">
-                        <span class="storyline-chapter-num">{{ loop.index }}</span>
+                        <span class="storyline-chapter-num">{{ chapter.tag }}</span>
                         <span class="storyline-chapter-body">
+                            <span class="storyline-chapter-kicker">{{ chapter.eyebrow }}</span>
                             <span class="storyline-chapter-title">{{ chapter.title }}</span>
                             <span class="storyline-chapter-summary">{{ chapter.summary }}</span>
                         </span>