feat: détection drift Sheets sur le dashboard (approche A)

- /api/dashboard/stats appelle Google Drive API pour récupérer modifiedTime du Sheets
- Compare avec createdAt de la dernière SyncRun → sheetDrift: boolean
- Dashboard affiche un bandeau amber cliquable si Sheets plus récent que la dernière sync

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

src/app/(dashboard)/page.tsx

@@ -23,6 +23,8 @@ interface LastSync {
 interface DashboardStats {
   products: ProductStats
   lastSync: LastSync | null
+  sheetDrift: boolean
+  sheetModifiedAt: string | null
 }
 
 function StatCard({
@@ -134,6 +136,22 @@ export default function DashboardPage() {
 
       <div className="p-6 flex flex-col gap-8 flex-1 overflow-y-auto">
 
+        {/* ── Alerte drift Sheets ───────────────────────────────── */}
+        {!statsLoading && stats?.sheetDrift && (
+          <Link href="/sync" className="flex items-center gap-3 bg-amber-950/40 border border-amber-600/50 rounded-xl px-4 py-3 hover:bg-amber-950/60 transition-colors group">
+            <span className="text-xl">⚠️</span>
+            <div className="flex-1 min-w-0">
+              <p className="text-sm font-semibold text-amber-300">Google Sheets modifié depuis la dernière sync</p>
+              <p className="text-xs text-amber-500 mt-0.5">
+                {stats.sheetModifiedAt
+                  ? `Dernière modification : ${formatDate(stats.sheetModifiedAt)}`
+                  : 'Des changements sont peut-être en attente de synchronisation'}
+              </p>
+            </div>
+            <span className="text-xs text-amber-400 group-hover:text-amber-300 whitespace-nowrap">Lancer la sync →</span>
+          </Link>
+        )}
+
         {/* ── KPIs ─────────────────────────────────────────────── */}
         <section>
           <h2 className="text-xs font-semibold text-slate-500 uppercase tracking-wider mb-3">Vue d&apos;ensemble</h2>

src/app/api/dashboard/stats/route.ts

@@ -43,18 +43,36 @@ async function fetchProductsLight(status: 'active' | 'draft'): Promise<LightProd
   return all
 }
 
+async function fetchSheetModifiedTime(): Promise<string | null> {
+  const apiKey = process.env.GOOGLE_API_KEY
+  const sheetId = process.env.GOOGLE_SHEETS_ID
+  if (!apiKey || !sheetId) return null
+  try {
+    const res = await fetch(
+      `https://www.googleapis.com/drive/v3/files/${sheetId}?fields=modifiedTime&key=${apiKey}`,
+      { cache: 'no-store' },
+    )
+    if (!res.ok) return null
+    const data = await res.json()
+    return (data.modifiedTime as string) ?? null
+  } catch {
+    return null
+  }
+}
+
 export async function GET() {
   const session = await getServerSession(authOptions)
   if (!session) return NextResponse.json({ error: 'Non authentifié' }, { status: 401 })
 
   try {
-    const [activeProducts, draftProducts, lastSyncRun] = await Promise.all([
+    const [activeProducts, draftProducts, lastSyncRun, sheetModifiedTime] = await Promise.all([
       fetchProductsLight('active'),
       fetchProductsLight('draft'),
       prisma.syncRun.findFirst({
         orderBy: { createdAt: 'desc' },
         include: { user: { select: { name: true } } },
       }),
+      fetchSheetModifiedTime(),
     ])
 
     const allProducts = [...activeProducts, ...draftProducts]
@@ -72,6 +90,12 @@ export async function GET() {
         }
       : null
 
+    // Drift : Sheets modifié après la dernière sync réussie
+    const sheetDrift =
+      sheetModifiedTime !== null &&
+      lastSyncRun !== null &&
+      new Date(sheetModifiedTime) > lastSyncRun.createdAt
+
     return NextResponse.json({
       products: {
         active: activeProducts.length,
@@ -80,6 +104,8 @@ export async function GET() {
         withoutImages,
       },
       lastSync,
+      sheetDrift,
+      sheetModifiedAt: sheetModifiedTime,
     })
   } catch (err) {
     const message = err instanceof Error ? err.message : 'Erreur inconnue'