fix: global BigInt → Number serialization for all Express JSON responses

Problem: BigInt fields (storageQuota, storageUsed, fileSize) crashed
Express with "Do not know how to serialize a BigInt" when returned
from res.json() — including Prisma query results.

Solution: Add BigInt.prototype.toJSON in index.ts (after all routes
registered, before server starts) so all JSON.stringify calls
automatically convert BigInt → Number.

Also fixed route handlers that manually returned BigInt fields.

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

packages/api/src/index.ts

@@ -3,6 +3,7 @@ import express from 'express';
 import cors from 'cors';
 import path from 'path';
 import cookieParser from 'cookie-parser';
+import { bigintToNumber } from './lib/prisma';
 
 import authRoutes from './routes/auth';
 import projectRoutes from './routes/projects';
@@ -51,6 +52,12 @@ app.use('/api/settings', settingsRoutes);
 app.use('/api/share', shareRoutes);
 app.use('/api/folders', folderRoutes);
 
+// ── BigInt-safe res.json() — patch Response prototype ─────────────────────────
+// Adding toJSON to BigInt prototype auto-converts BigInt → Number in JSON.stringify
+// This works because Express uses JSON.stringify internally for res.json()
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+(BigInt.prototype as any).toJSON = function () { return Number(this); };
+
 // ── 404 handler ─────────────────────────────────────────────────────────────
 app.use((_req, res) => {
   res.status(404).json({ error: 'Not found' });