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>

deep-research.php

@@ -129,6 +129,20 @@ require_once __DIR__ . '/includes/layout.php';
                         </div>
                     </div>
 
+                    <div id="drBranchPanel" class="branch-panel is-hidden" aria-label="Branch context">
+                        <div class="branch-panel__head">
+                            <span class="branch-panel__label">Branching from sub-question</span>
+                            <button type="button" id="drBranchClear" class="upload-clear">&times; Clear branch</button>
+                        </div>
+                        <p id="drBranchOrigin" class="branch-panel__origin"></p>
+                        <details class="branch-panel__prior">
+                            <summary>Prior brief summary</summary>
+                            <div id="drBranchSummary" class="branch-panel__brief"></div>
+                        </details>
+                        <label class="input-label" for="drBranchNotes">Add your notes / context</label>
+                        <textarea id="drBranchNotes" rows="3" placeholder="Optional: add observations, corrections, or additional context for this branch…"></textarea>
+                    </div>
+
                     <label class="input-label" for="drInput">Question or pasted text</label>
                     <textarea id="drInput" name="drInput" rows="8" placeholder="Describe the legal question, paste case notes, or both. The agent will research the corpus from 3–5 angles."></textarea>