feat: Corpus Intelligence page + timeline background events

Adds /corpus.php — a data transparency page showing what powers the
legal tools: 9 coverage categories with live doc counts, a full
sources table pulled from the corpus DB, the AI stack (LLMs, Whisper,
Qdrant, Azure AI Search, embeddings, chunking), and a pipeline flow
diagram. Stats are live via a new /api/corpus-stats.php endpoint
(queries dobetter_rag + bnl_admin). The reasoning sidebar is repurposed
as a Corpus health panel on this page.

Also ships the in-progress timeline background events toggle:
API and UI wired together via include_background param.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

includes/layout.php

@@ -16,6 +16,7 @@ $navItems = [
     'timeline'      => ['Timeline',      'Events'],
     'redact'        => ['Redact',        'Privacy'],
     'transcribe'    => ['Transcribe',    'Audio'],
+    'corpus'        => ['Corpus',        'Data & stack'],
 ];
 $toolName  = $toolName  ?? 'ask';
 $toolTitle = $toolTitle ?? 'Legal Tools';

includes/layout_footer.php

@@ -1,6 +1,9 @@
         </section><!-- /tool-panel -->
 
         <aside class="reasoning-panel" aria-labelledby="reasoningTitle">
+<?php if (!empty($reasoningPanelOverride)): ?>
+<?= $reasoningPanelOverride ?>
+<?php else: ?>
             <div class="reasoning-head">
                 <p class="eyebrow">Evidence trail</p>
                 <h2 id="reasoningTitle">Reasoning</h2>
@@ -14,6 +17,7 @@
                     </div>
                 </li>
             </ol>
+<?php endif; ?>
         </aside>
     </section><!-- /workspace -->
 </main><!-- /appShell -->