feat: migrate ESLint to v9 flat config

- Replace .eslintrc.json with eslint.config.mjs
- Simplify configuration using @antfu/eslint-config
- Add necessary ESLint plugin dependencies
- Disable overly strict style rules
- Set package.json type to module for ESM support
- Fix ESLint disable comment format

🤖 Generated with [Claude Code](https://claude.ai/code)

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

app/api/file-upload/route.ts

@@ -1,4 +1,4 @@
-import { type NextRequest } from 'next/server'
+import type { NextRequest } from 'next/server'
 import { client, getInfo } from '@/app/api/utils/common'
 
 export async function POST(request: NextRequest) {