feat(corpus): add save-to-corpus + private corpus search scope

- POST /api/save-to-corpus.php — saves tool output text to user's default CaveauAI corpus via ClientRagPipeline
- api/case/upload.php — dual-writes uploaded PDFs to CaveauAI client_documents (best-effort)
- assets/js/corpus-save.js — shared <dialog> handler for .js-save-corpus buttons on all tool pages
- includes/layout_footer.php — injects corpus-save.js + shared save dialog markup
- korrespond/deep-research/barnevernet/discrepancy JS — save-to-corpus buttons on output sections
- api/search.php + LegalTools::search() — corpus_scope param ('shared'|'private'|'both'), merges personal CaveauAI corpus with shared legal library when 'both'
- includes/tool_form.php + assets/js/tools.js — corpus scope radio toggle shown on search tab
- api/user-docs.php — add POST upload method for non-SSO authenticated users

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

assets/js/discrepancy.js

@@ -551,6 +551,17 @@
       els.results.appendChild(finalContainer.firstChild);
     }
 
+    // Save-to-corpus button (appended after final results)
+    const saveBtn = document.createElement('button');
+    saveBtn.type = 'button';
+    saveBtn.className = 'js-save-corpus secondary-button';
+    saveBtn.dataset.tool = 'discrepancy';
+    saveBtn.dataset.contentId = 'dcResults';
+    saveBtn.dataset.suggestedTitle = 'Discrepancy report';
+    saveBtn.textContent = 'Save to corpus';
+    saveBtn.style.marginTop = '16px';
+    els.results.appendChild(saveBtn);
+
     // Bind tabs
     els.results.querySelectorAll('.dc-tab').forEach((btn) => {
       btn.addEventListener('click', () => {