feat: implement backend API endpoints

- CSV upload with papaparse (handles date strings and Unix timestamps)
- Annotations CRUD (GET, POST, DELETE)
- Candles GET endpoint
- Export annotations as CSV

src/app/api/export/route.ts

@@ -0,0 +1,52 @@
+import { NextResponse } from 'next/server';
+import { db } from '@/lib/db';
+import { annotations, candles } from '@/lib/db/schema';
+import { eq } from 'drizzle-orm';
+
+export async function GET() {
+  try {
+    const allAnnotations = await db.select().from(annotations);
+
+    // Build CSV content
+    const csvRows = ['timestamp,label_type,price'];
+
+    for (const annotation of allAnnotations) {
+      let price: number | null = null;
+
+      if (annotation.label_type === 'break_up' || annotation.label_type === 'break_down') {
+        // For marker annotations, look up the candle's close price
+        const candleResult = await db
+          .select()
+          .from(candles)
+          .where(eq(candles.time, annotation.timestamp))
+          .limit(1);
+
+        if (candleResult.length > 0) {
+          price = candleResult[0].close;
+        }
+      } else if (annotation.label_type === 'line' && annotation.geometry) {
+        // For line annotations, use startPrice from geometry
+        const geometry = JSON.parse(annotation.geometry);
+        price = geometry.startPrice || null;
+      }
+
+      csvRows.push(
+        `${annotation.timestamp},${annotation.label_type},${price !== null ? price : ''}`
+      );
+    }
+
+    const csvContent = csvRows.join('\n');
+
+    return new NextResponse(csvContent, {
+      headers: {
+        'Content-Type': 'text/csv',
+        'Content-Disposition': 'attachment; filename="annotations.csv"',
+      },
+    });
+  } catch (error: any) {
+    return NextResponse.json(
+      { error: error.message || 'Failed to export annotations' },
+      { status: 500 }
+    );
+  }
+}