Add sub-question branching + document summary modals

- Source modal now shows LLM-generated document summary (lazy-gen + cached
  in documents.summary) instead of raw chunk text; toggle reveals matched
  chunk; "View all chunks" button fetches every chunk of the document via
  new api/document-chunks.php endpoint
- Each sub-question card gets a "Branch ↓" button that pre-fills the query
  with that sub-question and shows a context panel with the prior brief
  summary; prior_context + branch_notes are injected into interpretSeed()
  and synthesise() so the LLM knows where the research is coming from
- Upload document summaries generated at synthesis time and attached to
  upload sources alongside corpus summaries
- DB: documents.summary TEXT column added to bnl_corpus on chloe

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

api/document-chunks.php

@@ -0,0 +1,54 @@
+<?php
+declare(strict_types=1);
+
+require_once __DIR__ . '/../includes/bootstrap.php';
+
+dbnToolsRequireMethod('GET');
+dbnToolsRequireAuth();
+
+header('Content-Type: application/json; charset=utf-8');
+header('Cache-Control: no-store');
+
+try {
+    $documentId = isset($_GET['document_id']) ? (int)$_GET['document_id'] : 0;
+    if ($documentId <= 0) {
+        echo json_encode(['ok' => false, 'error' => 'document_id is required']);
+        exit;
+    }
+
+    $ragDb = dbnToolsRagDb();
+
+    $docStmt = $ragDb->prepare("SELECT id, title FROM documents WHERE id = ? LIMIT 1");
+    $docStmt->execute([$documentId]);
+    $doc = $docStmt->fetch(PDO::FETCH_ASSOC);
+    if (!$doc) {
+        echo json_encode(['ok' => false, 'error' => 'Document not found']);
+        exit;
+    }
+
+    $chunkStmt = $ragDb->prepare("
+        SELECT chunk_index, section_title, content
+        FROM chunks
+        WHERE document_id = ?
+        ORDER BY chunk_index ASC
+    ");
+    $chunkStmt->execute([$documentId]);
+    $chunks = [];
+    foreach ($chunkStmt as $row) {
+        $chunks[] = [
+            'chunk_index'   => (int)$row['chunk_index'],
+            'section_title' => $row['section_title'] ?? null,
+            'content'       => (string)$row['content'],
+        ];
+    }
+
+    echo json_encode([
+        'ok'       => true,
+        'document' => ['id' => (int)$doc['id'], 'title' => (string)$doc['title']],
+        'chunks'   => $chunks,
+    ], JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES);
+
+} catch (Throwable $e) {
+    error_log('DBN document-chunks error: ' . $e->getMessage());
+    echo json_encode(['ok' => false, 'error' => 'Internal error']);
+}