feat(l7): diff AI baseline vs current VTT in QCDetail

VttDiffView component (frontend/src/components/VttEditor/VttDiffView.tsx):
- Lazy-loads VTT version list (newest-first) and diffs version 1 (AI baseline)
  against the latest version
- Renders unified diff: green lines = added, red lines = removed (unchanged hidden)
- Collapsed by default; expand with "↔ Diff vs AI baseline" button
- Shows +N/-N change summary in header

QCDetail integration:
- VttDiffView added below both Captions and Audio Description VttEditors
  (only appears for the selected language)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/VttEditor/VttDiffView.tsx

@@ -0,0 +1,104 @@
+import { useState } from 'react';
+import { useQuery } from '@tanstack/react-query';
+import { apiClient } from '../../lib/api';
+import type { VttKind } from '../../types/api';
+
+interface VttDiffViewProps {
+  jobId: string;
+  lang: string;
+  kind: VttKind;
+}
+
+export function VttDiffView({ jobId, lang, kind }: VttDiffViewProps) {
+  const [open, setOpen] = useState(false);
+
+  const { data: versions } = useQuery({
+    queryKey: ['vtt-versions', jobId, lang, kind],
+    queryFn: () => apiClient.listVttVersions(jobId, lang, kind, 0, 200),
+    enabled: open,
+    staleTime: 30_000,
+  });
+
+  const versionList = versions?.versions ?? [];
+  const baseline = versionList.length > 0 ? versionList[versionList.length - 1] : null; // oldest = version 1
+  const latest = versionList.length > 0 ? versionList[0] : null; // newest first
+
+  const canShowDiff = baseline && latest && baseline.version !== latest.version;
+
+  const { data: diffData, isLoading: diffLoading } = useQuery({
+    queryKey: ['vtt-diff', jobId, lang, kind, baseline?.version, latest?.version],
+    queryFn: () => apiClient.diffVttVersions(jobId, lang, kind, baseline!.version, latest!.version),
+    enabled: open && !!canShowDiff,
+    staleTime: 30_000,
+  });
+
+  if (!open) {
+    return (
+      <button
+        onClick={() => setOpen(true)}
+        className="text-xs px-2 py-1 text-indigo-600 border border-indigo-200 rounded hover:bg-indigo-50"
+      >
+        ↔ Diff vs AI baseline
+      </button>
+    );
+  }
+
+  return (
+    <div className="border border-indigo-200 rounded-lg overflow-hidden">
+      <div className="flex items-center justify-between px-4 py-2 bg-indigo-50 border-b border-indigo-200">
+        <div className="flex items-center gap-3">
+          <span className="text-sm font-medium text-indigo-900">Diff vs AI baseline</span>
+          {diffData && (
+            <span className="text-xs text-gray-500">
+              <span className="text-green-600">+{diffData.added_count}</span>
+              {' / '}
+              <span className="text-red-500">-{diffData.removed_count}</span>
+              {` lines changed from v${baseline?.version} → v${latest?.version}`}
+            </span>
+          )}
+        </div>
+        <button
+          onClick={() => setOpen(false)}
+          className="text-xs text-indigo-500 hover:text-indigo-700"
+        >
+          Hide
+        </button>
+      </div>
+
+      <div className="max-h-72 overflow-y-auto font-mono text-xs bg-white">
+        {diffLoading && (
+          <div className="p-4 text-gray-400 text-center">Loading diff…</div>
+        )}
+
+        {!diffLoading && !canShowDiff && versionList.length > 0 && (
+          <div className="p-4 text-gray-400 text-center">No changes from baseline — content is identical to AI output.</div>
+        )}
+
+        {!diffLoading && versionList.length === 0 && (
+          <div className="p-4 text-gray-400 text-center">No version history yet.</div>
+        )}
+
+        {diffData?.lines.map((line, i) => {
+          if (line.type === 'unchanged') return null;
+          return (
+            <div
+              key={i}
+              className={`flex px-2 py-0.5 ${
+                line.type === 'added' ? 'bg-green-50 text-green-800' : 'bg-red-50 text-red-800'
+              }`}
+            >
+              <span className="w-6 shrink-0 select-none text-gray-400">
+                {line.type === 'added' ? '+' : '-'}
+              </span>
+              <span className="whitespace-pre-wrap break-all">{line.content}</span>
+            </div>
+          );
+        })}
+
+        {diffData && diffData.added_count === 0 && diffData.removed_count === 0 && (
+          <div className="p-4 text-gray-400 text-center">No changes from AI baseline.</div>
+        )}
+      </div>
+    </div>
+  );
+}

frontend/src/routes/admin/QCDetail.tsx

@@ -11,6 +11,7 @@ import {
 } from '../../hooks/useAccessibleVideoEdit';
 import { StatusBadge } from '../../components/StatusBadge';
 import { VttEditor } from '../../components/VttEditor/VttEditor';
+import { VttDiffView } from '../../components/VttEditor/VttDiffView';
 import { VideoWithCaptions } from '../../components/VideoWithCaptions';
 import { VoiceSelector } from '../../components/VoiceSelector';
 import { TimelinePreview } from '../../components/TimelinePreview';
@@ -1584,6 +1585,9 @@ export function QCDetail() {
                 glossaryTerms={glossaryTerms}
                 language={selectedLanguage}
               />
+              <div className="mt-2">
+                <VttDiffView jobId={id!} lang={selectedLanguage} kind="captions" />
+              </div>
             </div>
           )}
 
@@ -1611,6 +1615,9 @@ export function QCDetail() {
                 glossaryTerms={glossaryTerms}
                 language={selectedLanguage}
               />
+              <div className="mt-2">
+                <VttDiffView jobId={id!} lang={selectedLanguage} kind="ad" />
+              </div>
             </div>
           )}
         </div>