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>

api/search.php

@@ -17,5 +17,8 @@ dbnToolsWithTelemetry('search', $language, function () use ($input, $language):
     $asOfDate = isset($input['as_of_date']) && preg_match('/^\d{4}(-\d{2}(-\d{2})?)?$/', $input['as_of_date'])
         ? $input['as_of_date']
         : null;
-    return (new DbnLegalToolsService())->search($query, $language, $limit, $temporalMode, $asOfDate);
+    $scope = in_array($input['corpus_scope'] ?? '', ['shared', 'private', 'both'], true)
+        ? $input['corpus_scope']
+        : 'both';
+    return (new DbnLegalToolsService())->search($query, $language, $limit, $temporalMode, $asOfDate, $scope);
 });