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';