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>

timeline.php

@@ -42,6 +42,12 @@ require_once __DIR__ . '/includes/layout.php';
                         </div>
                         <p class="upload-hint" data-i18n="timelineConfidenceHint">Show all: includes events the model is uncertain about (shown in grey). Hide low-confidence: only returns events the model is reasonably sure of.</p>
 
+                        <div class="control-row" id="timelineBackgroundControl">
+                            <span class="control-label" data-i18n="timelineBackground">Background events</span>
+                            <label><input type="checkbox" id="includeBackgroundCheck" name="include_background" checked> <span data-i18n="timelineIncludeBackground">Include narrative / background dates</span></label>
+                        </div>
+                        <p class="upload-hint" data-i18n="timelineBackgroundHint">When checked, historical context dates are included (e.g. "born 30.07.2015", "met around 2011/2012"). Uncheck to extract only operational events and deadlines.</p>
+
                         <div class="control-row" id="timelineDatesControl">
                             <span class="control-label" data-i18n="timelineDates">Date types</span>
                             <label><input type="checkbox" id="includeRelativeCheck" name="include_relative" checked> <span data-i18n="timelineIncludeRelative">Include relative / recurring dates</span></label>