用 SSE 做流式 AI 输出：先出字，而不是先转圈

header('Content-Type: text/event-stream; charset=utf-8');
header('Cache-Control: no-cache');
header('X-Accel-Buffering: no');   // 关键：告诉 Nginx 不要缓冲这段响应

// 关掉 PHP 自身的输出缓冲，否则内容会攒到脚本结束才发
while (ob_get_level() > 0) {
    ob_end_flush();
}
ob_implicit_flush(true);

foreach ($chunks as $chunk) {
    echo 'data: ' . json_encode(['text' => $chunk], JSON_UNESCAPED_UNICODE) . "\n\n";
    flush();          // 推出去
}
echo "data: [DONE]\n\n";   // 显式结束，前端才知道"正常写完了"

const controller = new AbortController()

const response = await fetch('/admin/api/ai/generate', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify(payload),
  signal: controller.signal,           // 中断用的
})

const reader = response.body.getReader()
const decoder = new TextDecoder()
let buffer = ''

while (true) {
  const { done, value } = await reader.read()
  if (done) break
  buffer += decoder.decode(value, { stream: true })

  // SSE 以空行分隔事件；残缺的一段留在 buffer 等下一块
  const parts = buffer.split('\n\n')
  buffer = parts.pop() ?? ''
  for (const part of parts) {
    const line = part.replace(/^data: /, '')
    if (line === '[DONE]') return
    appendText(JSON.parse(line).text)
  }
}