feat: add WebDAV image storage

[lyon-le]

Title:
Add WebDAV image storage for generated images / 添加 WebDAV 图片存储

Body:

Summary / 概要

• Add optional WebDAV storage for generated images, with local-only, WebDAV-only, and local + WebDAV modes.
• 为生成图片新增可选 WebDAV 存储，支持仅本机、仅 WebDAV、本机 + WebDAV 三种模式。
• Add WebDAV connection testing and full local-to-WebDAV image sync from the settings page.
• 在设置页新增 WebDAV 连接测试，以及把已有本机图片全量同步到 WebDAV 的能力。
• Update image manager metadata to show whether each image exists locally, in WebDAV, or in both places.
• 图片管理页新增存储位置标记，可显示图片位于本机、WebDAV，或两端都有。
• Keep /images/..., thumbnails, downloads, deletes, and image task recovery working through the app when images are stored remotely.
• 图片远端存储后，仍通过应用保持 /images/... 访问、缩略图、下载、删除和图片任务恢复能力。
• Filter non-image files such as .DS_Store from the image index to avoid broken previews.
• 过滤 .DS_Store 等非图片文件，避免它们进入图片索引导致预览失败。

Related Issue / 关联 Issue

Refs #89

Verification / 验证

• uv run python -m unittest test.test_image_storage_service test.test_image_tasks_api test.test_generations_url
• uv run python -m compileall services api
• git diff --check