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/upload/route.ts

@@ -0,0 +1,134 @@
+import { NextRequest, NextResponse } from 'next/server';
+import Papa from 'papaparse';
+import { db } from '@/lib/db';
+import { candles } from '@/lib/db/schema';
+
+export async function POST(request: NextRequest) {
+  try {
+    const formData = await request.formData();
+    const file = formData.get('file') as File;
+
+    if (!file) {
+      return NextResponse.json(
+        { error: 'No file provided' },
+        { status: 400 }
+      );
+    }
+
+    const text = await file.text();
+
+    return new Promise((resolve) => {
+      Papa.parse(text, {
+        header: true,
+        dynamicTyping: true,
+        skipEmptyLines: true,
+        complete: async (results) => {
+          try {
+            const rows = results.data as any[];
+
+            // Validate headers
+            if (rows.length === 0) {
+              resolve(
+                NextResponse.json(
+                  { error: 'CSV file is empty' },
+                  { status: 400 }
+                )
+              );
+              return;
+            }
+
+            const firstRow = rows[0];
+            const requiredHeaders = ['time', 'open', 'high', 'low', 'close'];
+            const missingHeaders = requiredHeaders.filter(
+              (header) => !(header in firstRow)
+            );
+
+            if (missingHeaders.length > 0) {
+              resolve(
+                NextResponse.json(
+                  {
+                    error: `Missing required headers: ${missingHeaders.join(', ')}`,
+                  },
+                  { status: 400 }
+                )
+              );
+              return;
+            }
+
+            // Parse and prepare candle data
+            const candleData = rows.map((row) => {
+              let timestamp: number;
+
+              // Handle both date strings and Unix timestamps
+              if (typeof row.time === 'string') {
+                // Try parsing as date string
+                const date = new Date(row.time);
+                if (isNaN(date.getTime())) {
+                  throw new Error(`Invalid date format: ${row.time}`);
+                }
+                timestamp = Math.floor(date.getTime() / 1000);
+              } else if (typeof row.time === 'number') {
+                timestamp = row.time;
+              } else {
+                throw new Error(`Invalid time value: ${row.time}`);
+              }
+
+              return {
+                time: timestamp,
+                open: Number(row.open),
+                high: Number(row.high),
+                low: Number(row.low),
+                close: Number(row.close),
+              };
+            });
+
+            // Insert with upsert behavior (replace on conflict)
+            let count = 0;
+            for (const candle of candleData) {
+              await db
+                .insert(candles)
+                .values(candle)
+                .onConflictDoUpdate({
+                  target: candles.time,
+                  set: {
+                    open: candle.open,
+                    high: candle.high,
+                    low: candle.low,
+                    close: candle.close,
+                  },
+                });
+              count++;
+            }
+
+            resolve(
+              NextResponse.json({
+                success: true,
+                count,
+              })
+            );
+          } catch (error: any) {
+            resolve(
+              NextResponse.json(
+                { error: error.message || 'Failed to process CSV data' },
+                { status: 400 }
+              )
+            );
+          }
+        },
+        error: (error) => {
+          resolve(
+            NextResponse.json(
+              { error: `CSV parsing error: ${error.message}` },
+              { status: 400 }
+            )
+          );
+        },
+      });
+    });
+  } catch (error: any) {
+    return NextResponse.json(
+      { error: error.message || 'Internal server error' },
+      { status: 500 }
+    );
+  }
+}