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/deep-research.php

@@ -62,6 +62,8 @@ try {
     if (mb_strlen($advocateRole, 'UTF-8') > 200) {
         throw new DbnToolsHttpException('advocate_role is too long.', 422, 'advocate_role_too_long');
     }
+    $priorContext = is_array($input['prior_context'] ?? null) ? $input['prior_context'] : null;
+    $branchNotes  = mb_substr(trim((string)($input['branch_notes'] ?? '')), 0, 1000, 'UTF-8');
 
     if (mb_strlen($seedQuery, 'UTF-8') > 4000) {
         throw new DbnToolsHttpException('Query is too long.', 422, 'query_too_long');
@@ -118,7 +120,9 @@ try {
         $language,
         $controls,
         $emit,
-        $advocateRole
+        $advocateRole,
+        $priorContext,
+        $branchNotes
     );
 
     $result['ok'] = true;