feat: phase 1 of AI pipeline hardening — single LLM client + tier-aware models

Implements phase 1 of AI_PIPELINE_HARDENING_PLAN.md. Every Anthropic call
now goes through one module that owns retry, timeout, abort, structured-
output parsing, schema validation, and best-effort call telemetry.

* src/lib/llm.js — single callLLM entry point. Resolves model per tier
  (fast / standard / reasoning) with admin:model legacy fallback for the
  standard tier; 60s default timeout via AbortController; balanced-brace
  JSON extraction; LLMHttpError, LLMTruncatedError, LLMOutputError, and
  LLMValidationError surface clearly distinct failure modes.
* src/lib/llmRetry.js — exponential backoff with full jitter, retries
  only on transient HTTP statuses, honours Retry-After up to 60s, never
  retries on AbortError.
* src/lib/llmSchemas.js — Zod schemas for every structured task plus
  normalizeHandbookResult (collapses legacy "executes" relations into
  the canonical "executed_by" vocabulary).
* src/lib/api.js — thin shim over callLLM so existing callers (extraction
  pipeline, learning, quiz, R42, knowledge graph) keep working unchanged.
* src/lib/__tests__/ — 32 Vitest cases covering parse paths, error
  surfaces, simulation mode, model resolution, and schema validation.
* src/pages/Admin/index.jsx — three model inputs (fast / standard /
  reasoning) replacing the single legacy field; legacy value falls back
  for the standard tier so existing overrides survive.

Adds Zod and Vitest, plus an "npm run test" script.

Also cleans up the pre-existing repo-wide ESLint failures so phase 1's
"npm run lint passes" acceptance criterion can be checked: drops unused
React imports across the JSX tree (React 19 JSX runtime auto-imports),
attaches cause to rethrown errors in the service modules, ignores
pb_migrations in the ESLint config (PocketBase JSVM globals), and
removes one dead handleCreateCustom function in Leren.jsx. A real
behaviour bug surfaced in Testen.jsx — the quiz timer captured a stale
finishQuiz via setInterval closure; now updated via finishQuizRef so the
timer always invokes the latest callback.

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

src/components/admin/ContentManager.jsx

@@ -1,9 +1,8 @@
-import React, { useState, useEffect } from 'react';
+import { useState, useEffect } from 'react';
 import {
   RefreshCw, Wand2, Trash2, CheckCircle, Loader,
   AlertCircle, BookOpen, ArrowLeft, Eye
 } from 'lucide-react';
-import { motion, AnimatePresence } from 'framer-motion';
 import { getAllGeneratedContent, generateLearningContent, refineLearningContent, deleteCachedContent } from '../../lib/learningService';
 import LearningContentViewer from '../ui/LearningContentViewer';
 import Card from '../ui/Card';

src/components/admin/CurriculumManager.jsx

@@ -1,5 +1,5 @@
-import React, { useState, useEffect, useMemo } from 'react';
-import { Calendar, Wand2, ChevronDown, ChevronRight, RotateCcw, CheckCircle2, BookOpen, Loader, AlertTriangle } from 'lucide-react';
+import { useState, useEffect, useMemo } from 'react';
+import { Calendar, Wand2, ChevronDown, ChevronRight, RotateCcw, CheckCircle2, Loader, AlertTriangle } from 'lucide-react';
 import Card from '../ui/Card';
 import Button from '../ui/Button';
 import Tag from '../ui/Tag';
@@ -10,7 +10,6 @@ import {
   getQuarterForWeek,
   getQuarterName,
   getFullCurriculum,
-  hasCurriculum,
 } from '../../lib/curriculumService';
 
 const QUARTER_COLORS = {

src/components/admin/KnowledgeGraph.jsx

@@ -1,4 +1,4 @@
-import React, { useCallback, useEffect, useRef, useState } from 'react';
+import { useCallback, useEffect, useRef, useState } from 'react';
 import * as d3 from 'd3';
 import { Trash2, Edit2, Save, X, RefreshCw, AlertCircle, Plus, Link as LinkIcon } from 'lucide-react';
 import * as db from '../../lib/db';

src/components/admin/SuggestionsQueue.jsx

@@ -1,4 +1,4 @@
-import React, { useEffect, useState } from 'react';
+import { useEffect, useState } from 'react';
 import { Check, X, Clock, Sparkles } from 'lucide-react';
 import { kbStore } from '../../lib/kbStore';
 import Button from '../ui/Button';

src/components/admin/TeamManager.jsx

@@ -1,5 +1,5 @@
-import React, { useState, useEffect } from 'react';
-import { Users, UserPlus, Trash2, Edit2, Shield, User, CheckCircle } from 'lucide-react';
+import { useState, useEffect } from 'react';
+import { UserPlus, Trash2, Edit2, Shield, User, CheckCircle } from 'lucide-react';
 import Card from '../ui/Card';
 import Button from '../ui/Button';
 import Input from '../ui/Input';

src/components/admin/TestManager.jsx

@@ -1,11 +1,10 @@
-import React, { useState, useEffect } from 'react';
-import { RefreshCw, Trash2, CheckCircle, Loader, AlertCircle, HelpCircle, ArrowLeft, Eye, ChevronDown, ChevronUp } from 'lucide-react';
+import { useState, useEffect } from 'react';
+import { RefreshCw, Trash2, CheckCircle, Loader, AlertCircle, HelpCircle, ArrowLeft, Eye } from 'lucide-react';
 import { forceGenerateTopicQuestions, getTopicQuestionBank, deleteQuestion } from '../../lib/testService';
 import * as db from '../../lib/db';
 import Card from '../ui/Card';
 import Button from '../ui/Button';
 import Tag from '../ui/Tag';
-import { motion, AnimatePresence } from 'framer-motion';
 
 const TestManager = () => {
   const [topics, setTopics] = useState([]);

src/components/admin/UploadZone.jsx

@@ -1,7 +1,6 @@
-import React, { useState, useRef } from 'react';
+import { useState, useRef } from 'react';
 import { UploadCloud, AlertCircle, CheckCircle } from 'lucide-react';
 import { processSourceText } from '../../lib/extractionPipeline';
-import * as db from '../../lib/db';
 import Card from '../ui/Card';
 import Button from '../ui/Button';