fix: convert Unix epoch seconds to Date before inserting annotation timestamp

src/app/api/annotations/route.ts

@@ -60,18 +60,27 @@ export async function POST(request: NextRequest) {
       );
     }
 
+    // timestamp from client is Unix epoch seconds; convert to Date for Postgres
+    const timestampDate = typeof timestamp === 'number'
+      ? new Date(timestamp * 1000)
+      : new Date(timestamp);
+
     const result = await db
       .insert(annotations)
       .values({
         chart_id,
-        timestamp,
+        timestamp: timestampDate,
         label_type,
         geometry: geometry || null,
         color: color || '#3b82f6',
       })
       .returning();
 
-    return NextResponse.json(result[0], { status: 201 });
+    const row = result[0];
+    return NextResponse.json({
+      ...row,
+      timestamp: row.timestamp instanceof Date ? Math.floor(row.timestamp.getTime() / 1000) : row.timestamp,
+    }, { status: 201 });
   } catch (error: any) {
     return NextResponse.json(
       { error: error.message || 'Failed to create annotation' },