Add Deep Research tool — agent + rank/rerank RAG

New surface at /deep-research.php where the user pastes a question or
uploads PDF/DOCX/TXT case files and a LLM-orchestrated agent researches
the Do Better Norge legal corpus from 3-5 angles, with hybrid retrieval,
cross-encoder rerank, and synthesis that emits an inline-[n]-cited
markdown brief plus a numbered sources panel.

Uploaded documents are chunked + embedded in memory only (nomic-embed-text
via LiteLLM) and searched alongside the shared corpus during the same
request — never persisted to disk, DB, or Qdrant.

Reuses ClientRagPipeline::searchAll (hybrid + rerank), dbnV6 slice
helpers, and the existing extract.php text-extraction logic via a new
dbnToolsExtractUploadedFile() helper. Also adds dbnToolsCallGpuLlm()
helper in bootstrap.php — fixes a latent bug where LegalTools.php
was already calling that name with no definition.

Search.php is unchanged.

includes/layout.php

@@ -9,12 +9,13 @@ if (!dbnToolsIsAuthenticated()) {
 }
 
 $navItems = [
-    'ask'        => ['Ask',        'Source-grounded'],
-    'search'     => ['Search',     'Legal sources'],
-    'summarize'  => ['Summarize',  'Pasted text'],
-    'timeline'   => ['Timeline',   'Events'],
-    'redact'     => ['Redact',     'Privacy'],
-    'transcribe' => ['Transcribe', 'Audio'],
+    'ask'           => ['Ask',           'Source-grounded'],
+    'search'        => ['Search',        'Legal sources'],
+    'deep-research' => ['Deep research', 'Agent + RAG'],
+    'summarize'     => ['Summarize',     'Pasted text'],
+    'timeline'      => ['Timeline',      'Events'],
+    'redact'        => ['Redact',        'Privacy'],
+    'transcribe'    => ['Transcribe',    'Audio'],
 ];
 $toolName  = $toolName  ?? 'ask';
 $toolTitle = $toolTitle ?? 'Legal Tools';