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>

assets/css/tools.css

@@ -2573,7 +2573,7 @@ p {
 .dr-source-modal__meta dt:first-of-type { margin-top: 0; }
 
 .dr-source-modal__text {
-  white-space: pre-wrap;
+  white-space: normal;
   line-height: 1.7;
   color: var(--ink);
 }
@@ -2600,7 +2600,7 @@ p {
 
 .dr-subq-report__head {
   display: grid;
-  grid-template-columns: auto 1fr;
+  grid-template-columns: auto 1fr auto;
   gap: 10px;
   align-items: start;
   margin-bottom: 10px;
@@ -3213,3 +3213,166 @@ a.dr-source-title-link:hover {
   .adv-role-select, .adv-role-custom { max-width: 100%; }
   .adv-banner { flex-direction: column; align-items: flex-start; }
 }
+
+/* ── Branch-from-sub-question panel ──────────────────────────────────────── */
+.branch-panel {
+  background: #f0faf8;
+  border: 1px solid var(--teal);
+  border-left: 4px solid var(--teal);
+  border-radius: 8px;
+  padding: 14px 16px;
+  margin-bottom: 16px;
+}
+
+.branch-panel__head {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin-bottom: 6px;
+}
+
+.branch-panel__label {
+  font-weight: 700;
+  font-size: 0.82rem;
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+  color: var(--teal-dark);
+}
+
+.branch-panel__origin {
+  margin: 0 0 8px;
+  font-size: 0.88rem;
+  color: var(--ink);
+  line-height: 1.45;
+}
+
+.branch-panel__prior {
+  margin-bottom: 10px;
+  font-size: 0.87rem;
+}
+
+.branch-panel__prior summary {
+  cursor: pointer;
+  color: var(--teal-dark);
+  font-weight: 600;
+  user-select: none;
+}
+
+.branch-panel__brief {
+  margin-top: 6px;
+  padding: 8px 10px;
+  background: rgba(255,255,255,0.7);
+  border-radius: 5px;
+  font-size: 0.86rem;
+  color: var(--muted);
+  line-height: 1.5;
+  white-space: pre-wrap;
+}
+
+/* "Branch ↓" button on sub-Q cards */
+.dr-branch-btn {
+  align-self: start;
+  padding: 4px 10px;
+  font-size: 0.78rem;
+  font-weight: 700;
+  background: transparent;
+  border: 1px solid var(--teal);
+  border-radius: 5px;
+  color: var(--teal-dark);
+  white-space: nowrap;
+  transition: background 0.12s, color 0.12s;
+}
+
+.dr-branch-btn:hover {
+  background: var(--teal);
+  color: #fff;
+}
+
+/* ── Source modal: summary + chunk toggle + all-chunks ───────────────────── */
+.dr-modal-summary {
+  font-size: 0.93rem;
+  line-height: 1.65;
+  color: var(--ink);
+  margin-bottom: 14px;
+}
+
+.dr-modal-summary--empty em {
+  color: var(--muted);
+  font-size: 0.87rem;
+}
+
+.dr-modal-chunk-toggle,
+.dr-modal-all-chunks {
+  display: inline-block;
+  margin-bottom: 8px;
+  padding: 4px 10px;
+  font-size: 0.8rem;
+  font-weight: 600;
+  background: transparent;
+  border: 1px solid var(--line);
+  border-radius: 5px;
+  color: var(--teal-dark);
+  cursor: pointer;
+  transition: background 0.12s, border-color 0.12s;
+}
+
+.dr-modal-chunk-toggle:hover,
+.dr-modal-all-chunks:hover {
+  background: var(--soft-teal);
+  border-color: var(--teal);
+}
+
+.dr-modal-chunk-text {
+  white-space: pre-wrap;
+  font-size: 0.87rem;
+  line-height: 1.65;
+  color: var(--muted);
+  background: #f8f9fb;
+  border: 1px solid var(--line);
+  border-radius: 6px;
+  padding: 10px 12px;
+  margin-bottom: 12px;
+}
+
+.dr-modal-all-chunks {
+  display: block;
+  width: 100%;
+  text-align: left;
+  margin-top: 4px;
+}
+
+.dr-modal-chunks-list {
+  margin-top: 4px;
+}
+
+.dr-modal-chunks-head {
+  font-weight: 700;
+  font-size: 0.82rem;
+  color: var(--teal-dark);
+  padding: 6px 0 8px;
+  border-bottom: 1px solid var(--line);
+  margin-bottom: 8px;
+}
+
+.dr-modal-chunk-item {
+  padding: 8px 0;
+  border-bottom: 1px solid var(--line);
+}
+
+.dr-modal-chunk-item:last-child { border-bottom: 0; }
+
+.dr-modal-chunk-idx {
+  display: block;
+  font-size: 0.78rem;
+  font-weight: 700;
+  color: var(--coral);
+  margin-bottom: 4px;
+}
+
+.dr-modal-chunk-preview {
+  margin: 0;
+  font-size: 0.86rem;
+  line-height: 1.5;
+  color: var(--muted);
+  white-space: pre-wrap;
+}