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/korrespond.js

@@ -543,6 +543,12 @@
             </div>
           </div>
           <pre class="korr-draft-body" id="korrDraftNo">${esc(draftNo)}</pre>
+          <button type="button" class="js-save-corpus secondary-button"
+            data-tool="korrespond"
+            data-content-id="korrDraftNo"
+            data-suggested-title="${esc((data.output_type || 'Brev') + ' — ' + (data.recipient_body || ''))}">
+            Save to corpus
+          </button>
         </div>
         ${isSameLang ? '' : `
         <div class="korr-draft-col">
@@ -554,6 +560,12 @@
             </div>
           </div>
           <pre class="korr-draft-body" id="korrDraftUser">${esc(draftUser)}</pre>
+          <button type="button" class="js-save-corpus secondary-button"
+            data-tool="korrespond"
+            data-content-id="korrDraftUser"
+            data-suggested-title="${esc((data.output_type || 'Brev') + ' — ' + (data.recipient_body || '') + ' (translation)')}">
+            Save to corpus
+          </button>
         </div>`}
       </div>