feat(bsr): fetch BSR.xlsx from Dropbox instead of local file

- Rewrite api/fetch-bsr.ts to proxy BSR.xlsx from the Dropbox URL,
  matching the same pattern as fetch-buybox.ts and other data sources
- Add /api/fetch-bsr proxy in vite.config.ts for local dev

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

api/fetch-bsr.ts

@@ -1,27 +1,38 @@
-import { promises as fs } from 'fs';
-import path from 'path';
 import type { VercelRequest, VercelResponse } from '@vercel/node';
 
-export default async function handler(req: VercelRequest, res: VercelResponse) {
-    try {
-        const filePath = path.join(process.cwd(), 'BSR.xlsx');
+const BSR_DROPBOX_URL = "https://www.dropbox.com/scl/fi/r32r9x6r7bvgg6ckiaowk/BSR.xlsx?rlkey=vka50xyhc637riq901qo5bk16&st=y6ys2oz8&dl=1";
 
-        try {
-            await fs.access(filePath);
-        } catch {
-            return res.status(404).json({ error: 'BSR data file not found' });
+export default async function handler(req: VercelRequest, res: VercelResponse) {
+    // CORS headers
+    res.setHeader('Access-Control-Allow-Origin', '*');
+    res.setHeader('Access-Control-Allow-Methods', 'GET, OPTIONS');
+    res.setHeader('Access-Control-Allow-Headers', 'Content-Type');
+
+    if (req.method === 'OPTIONS') {
+        return res.status(200).end();
+    }
+
+    try {
+        console.log('[fetch-bsr] Fetching BSR.xlsx from Dropbox...');
+        const response = await fetch(BSR_DROPBOX_URL, {
+            cache: 'no-store',
+            headers: {
+                'Pragma': 'no-cache',
+                'Cache-Control': 'no-cache'
+            }
+        });
+
+        if (!response.ok) {
+            throw new Error(`Dropbox responded with ${response.status}`);
         }
 
-        const fileBuffer = await fs.readFile(filePath);
+        const buffer = await response.arrayBuffer();
+        console.log('[fetch-bsr] Successfully fetched BSR Excel, size:', buffer.byteLength);
 
         res.setHeader('Content-Type', 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
-        res.setHeader('Content-Disposition', 'attachment; filename="BSR.xlsx"');
-        res.setHeader('Cache-Control', 'public, max-age=3600');
-
-        // Convert Buffer to ArrayBuffer before sending for fetch compatibility on client
-        res.send(fileBuffer);
-    } catch (error) {
-        console.error('Error serving BSR Excel file:', error);
-        res.status(500).json({ error: 'Failed to serve BSR file' });
+        res.status(200).send(Buffer.from(buffer));
+    } catch (error: any) {
+        console.error('[fetch-bsr] Error:', error);
+        res.status(500).json({ error: error.message });
     }
 }