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>

includes/tool_form.php

@@ -19,6 +19,13 @@
                         <input type="number" id="numSpeakersInput" name="num_speakers" min="2" max="10" placeholder="auto" class="num-speakers-input" aria-label="Expected speaker count">
                     </div>
 
+                    <div class="control-row is-hidden" id="corpusScopeControl">
+                        <span class="control-label">Search</span>
+                        <label><input type="radio" name="corpusScope" value="both" checked> Legal Library + My Docs</label>
+                        <label><input type="radio" name="corpusScope" value="shared"> Legal Library only</label>
+                        <label><input type="radio" name="corpusScope" value="private"> My Docs only</label>
+                    </div>
+
                     <div class="control-row is-hidden" id="redactionControl">
                         <span class="control-label">Mode</span>
                         <label><input type="radio" name="redactionMode" value="standard" checked> Standard</label>