feat: phase 5 of AI pipeline hardening — R42 retrieval & telemetry

- Add dependency-free TF-IDF retrieval (src/lib/retrieval.js) with NL+EN
  stopwords and a WeakMap-cached index.
- Rewrite buildKbContext to ship the top-K relevant topics + verbatim-
  mentioned ids only, filter relations to the included set, and append a
  [kb_hash: <8 hex>] suffix so the ephemeral prompt cache busts when the
  graph changes. Returns { context, retrievedTopics, allTopics }.
- Add LOOKUP_TOPIC_TOOL and drive useChat through callLLM directly with a
  multi-hop tool_result loop capped at 3 hops; preserve Anthropic-provided
  tool_use ids through callLLM so the loop can echo correct tool_use_id.
- Truncate R42 history to the last 12 turns and prepend a single
  "(earlier conversation truncated)" assistant message.
- Set R42 chat defaults: temperature 0.3, maxTokens 2048.
- Add pb_migrations/1780500002_created_llm_calls.js (the best-effort
  logger in callLLM was already wired) and a new Admin → Diagnostics
  view showing the last 100 calls with token usage, cache-hit rate, and
  USD cost from a local Anthropic price table.
- Finalize AI_PIPELINE_HARDENING_PLAN.md: mark Phases 1–5 shipped and
  Phase 6 (eval harness) explicitly out of scope.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/pages/Admin/index.jsx

@@ -1,5 +1,5 @@
 import { useState, useEffect } from 'react';
-import { Database, FileText, Settings, Users, Network, Clock, CheckCircle2, AlertCircle, Save, Info, Layers, CheckSquare, CalendarDays } from 'lucide-react';
+import { Database, FileText, Settings, Users, Network, Clock, CheckCircle2, AlertCircle, Save, Info, Layers, CheckSquare, CalendarDays, Activity } from 'lucide-react';
 import Card from '../../components/ui/Card';
 import Tag from '../../components/ui/Tag';
 import Button from '../../components/ui/Button';
@@ -12,6 +12,7 @@ import ContentManager from '../../components/admin/ContentManager';
 import TestManager from '../../components/admin/TestManager';
 import TeamManager from '../../components/admin/TeamManager';
 import CurriculumManager from '../../components/admin/CurriculumManager';
+import Diagnostics from '../../components/admin/Diagnostics';
 import { Trash2 } from 'lucide-react';
 
 const TIER_PLACEHOLDERS = {
@@ -71,6 +72,7 @@ const Admin = () => {
     { key: 'curriculum', icon: CalendarDays, label: 'Curriculum' },
     { key: 'graph', icon: Network, label: 'Graph' },
     { key: 'team', icon: Users, label: 'Team' },
+    { key: 'diagnostics', icon: Activity, label: 'Diagnostics' },
     { key: 'settings', icon: Settings, label: 'Settings', bottom: true },
   ];
 
@@ -181,6 +183,14 @@ const Admin = () => {
           </div>
         )}
 
+        {activeTab === 'diagnostics' && (
+          <div className="animate-in fade-in duration-300 max-w-6xl mx-auto">
+            <h1 className="text-3xl text-teal mb-2">Diagnostics</h1>
+            <p className="text-fg-muted mb-8">LLM-aanroepen, tokenverbruik en geschatte kosten.</p>
+            <Diagnostics />
+          </div>
+        )}
+
         {activeTab === 'settings' && (
           <div className="animate-in fade-in duration-300 max-w-2xl">
             <h1 className="text-3xl text-teal mb-2">Settings</h1>