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/candles/route.ts

@@ -0,0 +1,20 @@
+import { NextResponse } from 'next/server';
+import { db } from '@/lib/db';
+import { candles } from '@/lib/db/schema';
+import { asc } from 'drizzle-orm';
+
+export async function GET() {
+  try {
+    const allCandles = await db
+      .select()
+      .from(candles)
+      .orderBy(asc(candles.time));
+
+    return NextResponse.json(allCandles);
+  } catch (error: any) {
+    return NextResponse.json(
+      { error: error.message || 'Failed to fetch candles' },
+      { status: 500 }
+    );
+  }
+}