feat: phase 3 of AI pipeline hardening — extraction quality

Replace stateless one-shot extraction with a stateful, paced, cancellable
pipeline. Six subtasks:

- 3.1 Sentence-aware chunking with 800-char overlap (was paragraph-only
  at 4000 chars). Hard-split fallback for runaway sentences.
- 3.2 Stateful extraction: chunks 2+ receive an "already-extracted topic
  IDs" hint capped at 200 IDs, so the model reuses IDs instead of
  inventing variants like software-developer vs software-engineer.
- 3.3 Token-bucket limiter in llmRetry.js (extractionLimiter, 5 req/min).
  callLLM awaits the limiter before fetch; 429+Retry-After calls
  pauseUntil. Replaces hard setTimeout(12000) and setTimeout(15000).
- 3.4 relevance_locked column on topics — admin edits to relevance are
  sticky across re-extraction. Migration + merge respects the flag +
  unlock checkbox in KnowledgeGraph edit form.
- 3.5 Unify relation vocabulary — handbook prompt no longer mentions
  legacy "executes"; one-shot migration rewrites existing executes rows
  to executed_by with source/target swapped.
- 3.6 Cancellation — Cancel button on UploadZone wired to an
  AbortController threaded into callLLM; aborted runs persist status =
  "cancelled" rather than "failed".

Tests: 16 new unit tests for chunkText, buildKnownIdsHint, and
createLimiter. All 61 tests pass, 0 lint errors, build clean.

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

src/components/admin/UploadZone.jsx

@@ -1,5 +1,5 @@
 import { useState, useRef } from 'react';
-import { UploadCloud, AlertCircle, CheckCircle } from 'lucide-react';
+import { UploadCloud, AlertCircle, CheckCircle, X } from 'lucide-react';
 import { processSourceText } from '../../lib/extractionPipeline';
 import Card from '../ui/Card';
 import Button from '../ui/Button';
@@ -12,24 +12,36 @@ const UploadZone = ({ onUploadComplete }) => {
   const [status, setStatus]                 = useState(null);
 
   const fileInputRef = useRef(null);
+  const abortRef     = useRef(null);
 
   // ── File upload (drag & drop / browse) ────────────────────────────────────
 
   const processFile = async (file) => {
     setIsProcessing(true);
     setStatus(null);
+    const controller = new AbortController();
+    abortRef.current = controller;
     try {
       const text = await file.text();
-      await processSourceText(text, file.name);
+      await processSourceText(text, file.name, { signal: controller.signal });
       setStatus({ type: 'success', msg: `Successfully processed: ${file.name}` });
       if (onUploadComplete) onUploadComplete();
     } catch (error) {
-      setStatus({ type: 'error', msg: `Error processing file: ${error.message}` });
+      if (error?.name === 'AbortError') {
+        setStatus({ type: 'error', msg: `Cancelled: ${file.name}` });
+      } else {
+        setStatus({ type: 'error', msg: `Error processing file: ${error.message}` });
+      }
     } finally {
+      abortRef.current = null;
       setIsProcessing(false);
     }
   };
 
+  const cancelProcessing = () => {
+    abortRef.current?.abort(new DOMException('Cancelled by user', 'AbortError'));
+  };
+
   const handleDragOver  = (e) => { e.preventDefault(); setIsDragging(true); };
   const handleDragLeave = ()  => setIsDragging(false);
 
@@ -80,6 +92,19 @@ const UploadZone = ({ onUploadComplete }) => {
         />
       </Card>
 
+      {/* Cancel sits outside the upload card so pointer-events-none doesn't disable it. */}
+      {isProcessing && (
+        <div className="flex justify-center">
+          <button
+            type="button"
+            onClick={cancelProcessing}
+            className="flex items-center gap-1 text-sm text-red-600 hover:text-red-700 underline"
+          >
+            <X size={14} /> Cancel extraction
+          </button>
+        </div>
+      )}
+
       {/* ─── Status messages ─── */}
       {status && (
         <div className={`p-4 rounded-[var(--r-sm)] flex items-start gap-3 ${