docs: add AI extract content length limit (4000 chars) (#809)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

vitepress-docs/docs/en/guide/feature/ai-extract.md

@@ -24,6 +24,10 @@ Extraction results are automatically saved to the `metadata` field in the databa
 | `ENABLE_AI_EMAIL_EXTRACT`  | Text/JSON | Whether to enable AI email recognition feature                                                                                   | `true`                           |
 | `AI_EXTRACT_MODEL`         | Text      | AI model name, choose from [models supporting JSON mode](https://developers.cloudflare.com/workers-ai/features/json-mode/#supported-models) | `@cf/meta/llama-3.1-8b-instruct` |
 
+## Content Length Limit
+
+To avoid AI model token limits, the maximum email content length for processing is **4000 characters**. Email content exceeding this limit will be truncated before AI analysis.
+
 ## Workers AI Binding
 
 Configure Workers AI binding in `wrangler.toml`:

vitepress-docs/docs/zh/guide/feature/ai-extract.md

@@ -24,6 +24,10 @@ AI 邮件识别功能使用 Cloudflare Workers AI 自动分析收到的邮件内
 | `ENABLE_AI_EMAIL_EXTRACT` | 文本/JSON | 是否启用 AI 邮件识别功能                                                                                                       | `true`                           |
 | `AI_EXTRACT_MODEL`        | 文本      | AI 模型名称，从[支持 JSON 模式的模型](https://developers.cloudflare.com/workers-ai/features/json-mode/#supported-models)中选择 | `@cf/meta/llama-3.1-8b-instruct` |
 
+## 内容长度限制
+
+为避免 AI 模型 token 限制，邮件内容最大处理长度为 **4000 字符**。超过此长度的邮件内容将被截断后再进行 AI 分析。
+
 ## Workers AI 绑定
 
 需要在 `wrangler.toml` 中配置 Workers AI 绑定：