Fix hardcoded localhost URL in FileURLService breaking Docker deployment

[Copilot]

FileURLService hardcoded http://localhost:8000 as default base_url, causing remote MinerU API to fail in Docker environments where localhost is unreachable.

Changes

• FileURLService.__init__: Remove hardcoded default, require FILE_SERVICE_BASE_URL env var
• get_file_service(): Delegate env var handling to __init__ instead of duplicating logic
• env.example: Update FILE_SERVICE_BASE_URL with clear "must configure" warning and placeholder value http://your-ip:8000 instead of localhost

Behavior

# Before: silently uses localhost (breaks in Docker)
service = FileURLService()  # base_url = "http://localhost:8000"

# After: fails fast with clear error if env var not set
service = FileURLService()
# ValueError: FILE_SERVICE_BASE_URL 环境变量未设置。
# 请在 .env 文件中配置 FILE_SERVICE_BASE_URL=http://your-ip:8000

Explicit base_url parameter still works for testing/override scenarios.

Original prompt

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This section details on the original issue you should resolve

<issue_title>File service URL hardcoded to localhost breaks Docker deployment</issue_title>
<issue_description>## 问题描述
文件服务 URL 硬编码为 localhost，导致 Docker 部署失败。在 Docker 环境中 localhost 无法访问，远程 MinerU API 无法获取文件。

受影响的文件

• src/file_url_service.py 行 21, 141

硬编码值

• 默认值为 "http://localhost:8000"

问题

虽然有正确的环境变量 FILE_SERVICE_BASE_URL，但被硬编码的默认值覆盖。

解决方案

应该移除硬编码的默认值，强制从环境变量读取。</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes File service URL hardcoded to localhost breaks Docker deployment #12

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[BukeLy]

没有在 .env.example 中添加示例 @copilot

[Copilot]

已更新 env.example，添加了 FILE_SERVICE_BASE_URL 的必须配置说明，并将默认值从 localhost 改为 your-ip 占位符，避免误用。(35e94f5)